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6
FOREWORD BY FABIEN COUSTEAU
8
INTRODUCTION OCEAN WATER THE PROPERTIES OF WATER
28 30
THE CHEMISTRY OF SEAWATER
32
TEMPERATURE AND SALINITY
34
LIGHT AND SOUND
36
OCEAN GEOLOGY
38
THE FORMATION OF EARTH
40
THE ORIGIN OF OCEANS AND CONTINENTS
42
THE EVOLUTION OF THE OCEANS
44
TECTONICS AND THE OCEAN FLOOR
48 52
OCEAN WINDS
54
SURFACE CURRENTS
58
UNDERWATER CIRCULATION
60
THE GLOBAL WATER CYCLE
64
OCEANS AND CLIMATE
66
EL NIÑO AND LA NIÑA
68
HURRICANES AND TYPHOONS
70
WAVES AND TIDES
74
OCEAN WAVES
76
TIDES
78
Malhotra
EDITOR Himani Khatreja
MANAGING EDITOR
ABOUT THIS BOOK
CIRCULATION AND CLIMATE
DK UK SENIOR EDITOR Peter
CONTENTS
LONDON, NEW YORK, MELBOURNE, MUNICH, AND DELHI
OCEAN ENVIRONMENTS COASTS AND THE SEASHORE COASTS AND SEA-LEVEL CHANGE COASTAL LANDSCAPES BEACHES AND DUNES
86 88 92 106
ESTUARIES AND LAGOONS
114
SALT MARSHES AND TIDAL FLATS
124
MANGROVE SWAMPS
130
SHALLOW SEAS
138
OCEAN LIFE INTRODUCTION TO OCEAN LIFE
204
CLASSIFICATION
206
CYCLES OF LIFE AND ENERGY
BRYOZOANS
305
ECHINODERMS
306
SMALL, BOTTOM-LIVING PHYLA
313
PLANKTONIC PHYLA
317
212
TUNICATES AND LANCELETS
318
SWIMMING AND DRIFTING
214
JAWLESS FISHES
320
CONTINENTAL SHELVES
140
ROCKY SEABEDS
142
SANDY SEABEDS
144
BOTTOM LIVING
216
SHARKS, RAYS, AND CHIMAERAS
322
SEAGRASS BEDS AND KELP FORESTS
ZONES OF OCEAN LIFE
218
BONY FISHES
336
146
OCEAN MIGRATIONS
220
REPTILES
368
CORAL REEFS
152
LIVING DOWN DEEP
222
BIRDS
378
164
BIOLUMINESCENCE
224
MAMMALS
400
THE HISTORY OF OCEAN LIFE
226
THE PELAGIC ZONE THE OPEN OCEAN AND OCEAN FLOOR
166
KINGDOMS OF OCEAN LIFE
230
ZONES OF THE OPEN OCEAN
168
BACTERIA AND ARCHAEA
232
SEAMOUNTS AND GUYOTS
174
CHROMISTS
234
THE CONTINENTAL SLOPE AND RISE
176
OCEAN-FLOOR SEDIMENTS
180
ABYSSAL PLAINS, TRENCHES, AND MID-OCEAN RIDGES
182
VENTS AND SEEPS
188
THE POLAR OCEANS
190
ICE SHELVES
192
ICEBERGS
194
SEA ICE
198
POLAR OCEAN CIRCULATION
200
ATLAS OF THE OCEANS
BROWN SEAWEEDS
238
OCEANS OF THE WORLD
PLANT LIFE
242
THE ARCTIC OCEAN
424
RED SEAWEEDS
244
THE ATLANTIC OCEAN
428
GREEN SEAWEEDS
246
THE INDIAN OCEAN
446
GREEN ALGAE
248
THE PACIFIC OCEAN
456
MOSSES
249
THE SOUTHERN OCEAN
482
FLOWERING PLANTS
250
422
FUNGI
254
GLOSSARY
488
ANIMAL LIFE
256
INDEX
494
SPONGES
258
ACKNOWLEDGMENTS
510
CNIDARIANS
260
FLATWORMS
271
RIBBON WORMS
273
SEGMENTED WORMS
274
MOLLUSKS
276
ARTHROPODS
290
6
About this Book
Ocean Environments
THIS BOOK IS DIVIDED INTO
four chapters. An overview of the physical and chemical features of the oceans is given in the introduction; ocean environments looks at the main zones of the oceans, and ocean life examines the life-forms that inhabit them. The atlas of the oceans contains detailed maps of the oceans. Most chapters are divided into smaller sections.
This chapter looks at specific parts of the oceans. It is divided into sections on different zones, starting with coasts and the seashore and then moving to progressively deeper waters, first with shallow seas and then the open ocean and ocean floor. A final section, polar oceans, looks at the frozen waters around the North and South Poles. In each section, explanatory pages describe typical features and formative processes, while the succeeding pages contain profiles of actual features. The profiles are arranged by geographical location, starting with the Arctic Ocean and followed, in order, by the Atlantic, Pacific, Indian, and Southern Oceans. SHALLOW SEAS
160
PACIFIC OCEAN WEST
Shiraho Reef
TYPE
Fringing reef
10 square km (4 square miles)
AREA
Reasonable; damaged in parts by bleaching in 1998, 2007
CONDITION
Southeast coast of Ishigaki Island, at the CORAL REEFSLOCATION 153
SHALLOW SEAS
152
southwestern extremity of Japanese archipelago
CORAL DIVERSITY
Coral Reefs
Introduction
Reef Formation
In this seascape off a Fijian island, groups of shoaling sea goldies hover over diverse species of coral, sponges, and other reef organisms.
The individual animals that make up corals are called polyps. The polyps of the main group of reef-building corals, stony corals, secrete limestone, building on the substrate underneath. The polyps also form colonies that create community skeletons in a variety of shapes. An important contributor to the life of these corals is the presence within the polyps of tiny organisms called zooxanthellae, which provide much of the polyps’ nutritional needs. Other organisms that add their skeletal remains to the reef include mollusks and echinoderms. Grazing and boring organisms also contribute, by breaking coral skeletons into sand, which fills gaps in the developing reef. Algae and other encrusting organisms help bind the sand and coral fragments together. Most reefs do not grow continuously but experience spurts of growth interspersed with quieter periods, which are sometimes associated with recovery from storm damage.
CORAL REEFS ARE SOLID STRUCTURES
built from the remains of small marine organisms, principally a group of colony-forming animals called stony (or hard) corals. Reefs cover about 108,000 square miles (280,000 square km) of the world’s shallow marine areas, growing gradually as the organisms that form their living surfaces multiply, spread, and die, adding their limestone skeletons to the reef. Coral reefs are among the most complex and beautiful of Earth’s ecosystems, and are home to a fantastic variety of animals and other organisms; but they are also among the most heavily utilized and economically valuable. Today, the world’s reefs are under pressure from numerous threats to their health.
This opening chapter is divided into four sections. In ocean water, the properties of water itself are examined. ocean geology covers the materials of the ocean floor and the way that it changes over time. circulation and climate is about the interaction between oceans and the atmosphere and the large-scale movement of water, while tides and waves looks at movements and disturbances of water on a smaller scale.
Shiraho Reef, off Ishigaki Island, part of the Japanese archipelago, came to notice in the 1980s as an outstanding example of biodiversity, with some 120 species of coral and 300 fish
coral grows on shoreline, forming fringing reef
BARRIER REEF
Stony corals can grow only in clear, sunlit, shallow water where the temperature is at least 64˚F (18˚C), and preferably 77–84˚F (25–29˚C). They grow best where the average salinity of the water is 36 ppt (parts per thousand) and there is little wave action or sedimentation from river runoff. These conditions occur only in some tropical and subtropical areas.The highest concentration of coral reefs is found in the Indo-Pacific region, which stretches from the Red Sea to the central Pacific. A smaller concentration of reefs occurs around the Caribbean Sea. In addition to warm-water reefs, awareness is growing about other corals that do not depend on sunlight, and form deep, cold-water reefs—some of them outside the tropics (see p.178).
PACIFIC OCEAN WEST
Nusa Tenggara TYPE Fringing reefs, barrier reefs
BARRIER REEF
FRINGING REEF
5,000 square km (2,000 square miles)
Nusa Tenggara is a chain of around 500 coral-fringed islands in southern Indonesia. The northern islands are volcanic in origin, while the southern islands consist mainly of uplifted coral limestone. Many of the reefs have been only rarely explored. However, what surveys have been carried out
Damaged by fishing practices
CONDITION
Southern Indonesia, from Lombok in the west to Timor in the east
LOCATION
COLD-WATER CORAL
This species, Lophelia pertusa, is one of a few of the reef-forming corals that grow in cold water, at depths to 1,650 ft (500 m).
spread of volcanic ash and gases into rain clouds
THE OCEANS CONTAIN MILLIONS OF DISSOLVED chemical substances. Most of these are present in exceedingly small concentrations. Those present in significant concentrations include sea salt, which is not a single substance but a mixture of charged particles called ions. Other constituents include gases such as oxygen and carbon dioxide. One reason the oceans contain so many dissolved substances is that water is an excellent solvent.
The Salty Sea
salts are leached from rocks into rivers and streams and flow to ocean
The salt in the oceans exists in the form of charged particles, called ions, some positively charged and some negatively charged. The most common of these are sodium and chloride ions, the components of ordinary table salt (sodium chloride). Together they make up about 85 percent by mass of all the salt in the sea. Nearly all the rest is made up of the next four most common ions, which are sulfate, magnesium, calcium, and potassium. All these ions, together with several others present in smaller quantities, exist throughout the oceans in fixed proportions. Each is distributed extremely uniformly—this is in contrast to some other dissolved substances in seawater, which are unevenly distributed.
Shown here are various sources, sinks, and exchange processes for the ions, salts, and minerals (yellow arrows), gases (pink arrows), and plant nutrients (turquoise arrows) in seawater.
volcanic ash drifts down to sea
Gases in Seawater
KEY gases
This section covers the properties of the water molecule, the chemistry of seawater, and the way that attributes such as temperature, pressure, and light transmission change with depth in ocean water.
The main gases dissolved in seawater are nitrogen (N), oxygen (O2 ), and carbon dioxide (CO2). The levels of O2 and CO2 vary in response to the activities of photosynthesizing organisms (phytoplankton) and animals. The level of O2 is generally highest near the surface, where the gas is absorbed from the air and also produced by photosynthesizers. Its concentration drops to a minimum in a zone between about 660 ft (200 m) and 3,300 ft (1,000 m), where oxygen is consumed by bacterial oxidation of dead organic matter and by animals feeding on this matter. Deeper down, the O2 level increases again. CO2 levels are highest at depth and lowest at the surface, where the gas is taken up by photosynthesizers faster than it is produced by respiration.
ions, salts, and minerals plant nutrients
salt spray onto land
CARBON SINK
nutrients from soil wash into rivers and streams, and flow to ocean washing of ions from volcanic dust and gases into sea, dissolved in rain
Many marine animals, such as nautiluses (below), use carbonate (a compound of carbon and oxygen) in seawater to make their shells. After they die, the shells may form sediments and eventually rocks.
dust blown off land exchange of gases between ocean and atmosphere
exchange of gases between animals and seawater
OXYGEN PRODUCER AND CONSUMER
Oxygen levels in the upper ocean depend on the balance between its production by photo-synthesizing organisms, such as kelp, and its consumption by animals, such as fish.
BREAKDOWN OF SALT
If 2½ gallons (10 liters) of seawater are evaporated, about 123/4 oz (354 g) of salts are obtained, of the types shown below. 2½ gallons (10 liters) of seawater
Nutrients
other salts 1/4 oz (7.5g) calcium sulfate (gypsum) 2/3oz (17.7g) magnesium salts 2oz (54.8g) sodium chloride (halite) 10oz (274g)
+
– –
– Na+
–
–
– –
+
– +
+ +
+ +
+ Cl–
+
+
+ –
+ + –
– chloride ion (negative charge)
uptake of nutrients by phytoplankton
nutrient upwelling exchange of gases between phytoplankton and seawater
–
–
–
water molecule
–
PEOPLE
ALEXANDER MARCET The Swiss chemist and doctor Alexander Marcet (1770–1822) carried out some of the earliest research in marine chemistry. He is best known for his discovery, in 1819, that all the main chemical ions in seawater (such as sodium, chloride, and magnesium ions) are present in exactly the same proportions throughout the world’s oceans. The unchanging ratio between the ions holds true regardless of any variations in the salinity of water and is known today as the principle of constant proportions.
Sources and Sinks
sinking and
release of minerals from hydrothermal vents
decomposition The ions that make up the salt in the oceans have arrived of dead there through various processes. Some were dissolved out of organisms rocks on land by the action of rainwater and carried to the sea in rivers. Others entered the sea in the emanations of hydrothermal vents (see p.188), in dust blown off the land, or came from volcanic ash. There are also “sinks” for every type of ion—processes that remove them from seawater. These range from salt spray onto land to the precipitation of various ions onto the seafloor as mineral deposits. Each type of ion has a characteristic residence time. This is the time that an ion remains in seawater before it is removed. The common ions in seawater have long residence times, ranging from a few hundred years to hundreds of millions of years.
The Origin of Oceans and Continents
OCEAN GEOLOGY ▶
polar easterly
Ocean Winds
polar-front jet stream—narrow ribbon of strong wind at high altitude at top of front
THE PATTERN OF AIR MOVEMENT
over the oceans results from solar heating of the atmosphere and Earth’s rotation. This pattern of winds is modified by linked areas of low and high pressure (cyclones and anticyclones), which continually move over the oceans’ surface. Near coasts, additional onshore and offshore breezes are common. These are caused by differences in the capacity of sea and land to absorb heat.
The atmospheric cells cause north–south air movements. These are altered by the Coriolis effect. As the Earth spins, parcels of air at different latitudes in the atmosphere have different west-to-east velocities (air at the Equator moves fastest). When they change latitude by moving to the north or south, they retain these west-to-east velocities, which differ from those of air in the AIR DEFLECTIONS In the Northern Hemisphere, latitudes they move into. Hence, the air veers to the Coriolis effect causes all air movements to be the east (in the direction deflected to the right of of Earth’s spin) when their initial direction. In moving away from the the Southern Hemisphere, Equator and to the west they veer to the left. when moving toward it.
air deflected to right
air deflected to left
initial direction of air movement
westerlies
polar northeasterlies
westerlies
northeasterly monsoon (Nov–Mar)
northeasterly trade winds
Tropic of Cancer
equator
Tropic of Capricorn
top layer of upper mantle
bombardment gradually erased
THE EARLY EARTH mantle vigorous convection cells in upper mantle
The oceanic crust has a higher density than the continental crust, making it less buoyant. Both types of crust can be thought of as floating on the “plastic” upper mantle, and the oceanic crust lies lower due to its lower buoyancy. It is relatively thin, with a depth of never more than 7 miles (11 km), compared with a thickness of 15–43 miles (25–70 km) for most continental crust. It consists mainly of basalt, an igneous rock that is low in silica compared with continental rocks, and richer in calcium than the mantle. Basalt lava is created when hot material in the upper mantle is decompressed, allowing it to melt and form liquid magma. The decompression occurs beneath rifts in the crust, such as those found at the mid-ocean ridges, and it is through these rifts that lava is extruded onto the surface to create new ocean crust.
westerlies
volcanic eruptions add gases and water vapor to atmosphere
MANTLE ROCKS
ANDEAN VOLCANOES
This radar image shows volcanoes formed from andesite lava, whose composition is intermediate between oceanic and continental rocks.
55
This section describes the large-scale circulation of the oceans, both deep down and at the surface. It also looks at ocean climates and the many ways that the oceans and the atmosphere influence one another.
northeasterly trade wind
SATELLITE IMAGING
air ascends from cyclone air descends into anticyclone low pressure at center
central area of high pressure cold air sinks
Prevailing Winds The winds produced by pressure differences and modified by the Coriolis effect are called the prevailing winds. In the tropics and subtropics, the air movements toward the equator in Hadley cells are deflected to the west. These are known as trade winds. They comprise the northeasterly trades in the Northern Hemisphere, and southeasterly trades in the south. At higher latitudes, the surface winds in Ferrel cells deflect to the east, producing the westerlies. In the Southern Hemisphere, these winds blow from west to east without meeting land. Those around latitudes of 40˚S are known as the Roaring Forties. In polar regions, winds deflect to the west as they move away from the poles. These are known as polar northeasterlies and southeasterlies.
southwesterly monsoon (Apr–Oct)
Pressure-system Winds
warm air rising
ASCAT antenna (one of three)
prevailing cool local warm local cool
air spirals around central area of low pressure
air moving from high to low pressure deflected by Coriolis effect to form spiral
cold air flows toward area of low pressure
In any area of ocean where air sinks—often at subtropical latitudes—a zone of high atmospheric pressure, or anticyclone, develops. Where warm air rises, areas of low pressure, called cyclones or depressions, occur. These often develop near the equator and subpolar latitudes. Cyclones and anticyclones create linked, circulating wind patterns, which continually move and change. In the Northern Hemisphere, there is a clockwise movement of air around an anticyclone, and a counterclockwise motion CYCLONES AND ANTICYCLONES around a cyclone. This pattern is reversed in the Air moves from an area of Southern Hemisphere. Local pressure systems can affect high pressure toward one the general pattern of prevailing winds. In particular, of low pressure, but the cyclones move swiftly over the ocean and can produce Coriolis effect modifies this, producing circular winds. rapid changes in wind strength and direction. warm air cools at high altitude
BREEZY COAST
77
Ocean Waves
Wave Generation
WAVES ARE DISTURBANCES
in the ocean that transmit energy from one place to another. The most familiar types of waves—the ones that cause boats to bob up and down on the open sea and dissipate as breakers on beaches—are generated by wind on the ocean surface. Other wave types include tsunamis, which are often caused by underwater earthquakes (see p.49), and internal waves, which travel underwater between water masses. Tides (see p.78) are also a type of wave.
Wave Properties
direction of wave motion
TIDES AND WAVES ▶ trough
Wind energy is imparted to the sea surface through friction and pressure, causing waves. As the wind gains strength, the surface develops gradually from flat and smooth through growing levels of roughness. First, ripples form, then larger waves, called chop. The waves continue to build, their maximum size depending on three factors: wind speed, wind duration, and the area over which the wind is blowing, called the fetch. When waves are as large as they can get under the current conditions of wind speed and size of fetch, the sea surface is said to be “fully developed.” The overall state of a sea surface can be summarized by the significant wave height—defined as the average height of the highest one-third of the waves. For example, in a fully developed sea produced by winds of about 25 mph (40 kph), the significant wave height is typically about 8 ft (2.5 m).
wave height (amplitude)
disordered sea surface in fetch area
wind direction
ripples turn to chop
PHYLUM Mollusca
CLASSES 8
SPECIES 73,683
Anatomy Most mollusks have a head, a soft body mass, and a muscular foot. The foot is formed from the lower body surface and helps it to move. Mollusks have what is called a hydrostatic skeleton—their bodies are supported by internal fluid pressure rather than a hard skeleton. All mollusks have a mantle, a body layer that covers the upper body and may or may not secrete a shell. The shell of bivalves (clams and relatives) has two halves joined by a hinge; these can be held closed by powerful muscles while the tide is out, or if danger threatens. Mollusks other than bivalves have a rasping mouthpart, or radula, which is unique to mollusks. Cephalopods (octopuses, squid, and cuttlefish) also have beaklike jaws as well as tentacles, but most lack a shell, while most gastropods (slugs and snails) have a single shell. This is usually a spiral in snails, but can be cone-shaped in other forms, such as limpets.
color-coded panel shows position of group being described (indicated with white outline) in the classification hierarchy
gill
outside the fetch, waves become sorted by speed and wavelength
wave shortens in length and decreases in speed but increases in height
muscular foot
wave finally breaks
radula hinge ligament
BIVALVE ANATOMY
Bivalves are housed within a shell of two halves (right) from which the siphons and muscular foot can be extended. The shell is opened and closed by the adductor muscles, labeled in the body plan (far right).
CHOPPY SEA
In a choppy sea, the waves are 4–20 in (10–50 cm) high and have a wavelength of 10–40 ft (3–12 m).
direction of wave advance fetch (area over which wind blows)
wave reaches critical ratio of height to length and begins to break
water motion caused by the wave begins to interact with the sea bed and slow down
FULLY DEVELOPED ROUGH SEA
Wind speeds over 40 mph (60 kph) can generate very rough seas with waves more than 10 ft (3 m) high.
path of individual water particle
PARTICLE MOVEMENT
Wave Propagation Interference between two or more large waves occasionally causes a giant or “rogue” wave. This one, recorded in the Atlantic Ocean in 1986, had an estimated height of 56 ft (17 m). It broke over the ship pictured, bending its foremast back by 20˚.
eye SPIRAL SNAIL SHELL
water motion occurs offshore to depth of half the wavelength
GROUP INTRODUCTION ▶
BUILDING WAVES
ROGUE WAVES
mantle cavity
digestive system
These tiny waves are just a few millimetres high and have a wavelength of under 1½ in (4 cm).
SWELL
A swell is a series of large, evenly spaced waves, often observed hundreds of miles away from the storm that spawned them. Wavelengths range from tens to hundreds of feet.
SHOALING AND BREAKING
Shoaling occurs as waves enter shallow water. The wave length and speed both decrease, but the wave gains height. When the crest gets too steep, it curls and breaks.
HUMAN IMPACT
RIDING THE WAVES When a swell reaches a suitably shaped beach, it can produce excellent surfing conditions. Small spilling breakers are ideal for novice surfers, while experts seek out large plunging breakers that form a “tube” they can ride along. For tube-riding, the break of the wave must progress smoothly either to the right or left. Here, a surfer rides a rightbreaking wave in Hawaii— it is breaking from left to right behind the surfer.
Arrival on Shore As waves approach a shore, the motion they generate at depth begins to interact with the sea floor. This slows the waves down and causes the crests in a series of waves to bunch up—an effect called shoaling. The period of the waves does not change, but they gain height as the energy each contains is compressed into a shorter horizontal distance, and eventually break. There are two main types of breaker. Spilling breakers occur on flatter shores: their crests break and cascade down the front as they draw near the shore, dissipating energy gradually. In a plunging breaker, which occurs on steeper shores, the crest curls and falls over the front of the advancing wave, and the whole wave then collapses at once. Waves can also refract as they reach a coastline. This concentrates wave energy onto headlands (see p.93) and shapes some types of beach (see p.106). WAVE REFRACTION
When waves enter a bay enclosed by headlands, they are refracted (bent) as different parts of the wave-front encounter shallow water and slow down.
INTRODUCTION
In the fetch, many different groups of waves of varying wavelength are generated and interfere. As they disperse away from the fetch, the waves become more regularly sized and spaced. This is because the speed of a wave in open water is closely related to its wavelength. The different groups of waves move at different speeds and so are naturally sorted by wavelength: the largest, fastest-moving waves at the fore, the smaller, slower-moving ones behind. This produces a regular wave pattern, or swell. Occasionally, groups of waves from separate storms interfere to produce unusually large “rogue” waves. As they propagate across the open ocean, wind-generated waves maintain a constant speed, which is unaffected by depth until they reach shallow water. Only with waves of extremely long wavelength—tsunamis—is the speed of propagation affected by water depth.
water carried up shore in swash zone
Pages such as the ones shown here describe groups of organisms in general. All introductions contain an account of the defining physical characteristics, usually followed by further information on behavior, habitats, and classification.
REEF-DWELLING GOLIATH
The tropical giant clam is the largest bivalve and may measure more than 3 ft (1 m) across and weigh over 440 lb (220 kg).
GASTROPOD ANATOMY
spiral shell
sensory tentacle
direction of wave motion
Within the wave-generation area, the sea surface is usually quite confused—the result of groups of waves of different size and wavelength interfering with each other. Outside this area, the waves become sorted by speed to produce a more regular pattern, called a swell.
crest
As waves pass over the surface, the particles of water do not move forward with the waves. Instead, they gyrate in little circles or loops. Underwater, the particles move in ever-smaller loops. At a depth below about half the distance between crests, they are quite still.
AMONG THE MOST SUCCESSFUL of all marine animals, mollusks display great diversity and a remarkable range of body forms, allowing them to live almost everywhere from the ocean depths to the splash zone. They include oysters, sea slugs, and octopuses. Most species have shells SPECIES 73,683 and are passive or slow-moving; some lack eyes. Others are intelligent, active hunters with complex nervous systems and large eyes. Filter-feeding mollusks, such as clams, are crucial to coastal ecosystems, as they provide food for other animals and improve water quality and clarity. Many mollusks are commercially important for food, pearls, and their shells. DOMAIN Eucarya
KINGDOM Animalia
CAPILLARY WAVES (RIPPLES)
A group of waves consists of several crests separated by troughs. The height of the waves is called the amplitude, the distance between successive wave crests is known as the wavelength, and the time between successive wave crests is the period. Waves are classified into types based on their periods. They range from ripples, which have periods of less than 0.5 seconds, up to tsunamis and tides, whose periods are measured in minutes and hours (their wavelengths range from hundreds to thousands of miles). In between these extremes are chop and swell—the most familiar types of surface wave. Ocean waves behave like light rays: they are reflected or refracted by obstacles they encounter, such as islands. When different wave groups meet, they interfere—adding to, or canceling, each other. wavelength
PLUNGING BREAKER
“Barrel” or “tube-forming” breakers like this occur when the waves reaching shore have large amounts of energy. The seabed must be firm and quite steep.
ANIMAL LIFE
Mollusks
PHYLUM Mollusca CLASSES 8
DAY AND NIGHT
WAVES AND TIDES
still-water level
276
Land heats up faster
Local winds, called onshore and offshore than water during the day. air heats up Warm air rises over the land and rises over cool air breezes, are generated near coasts, especially in land drawn in and draws in cold air from sunny climes. Onshore breezes—sometimes the sea. At night, the land called sea breezes—develop during the day. cools more quickly, These are caused by the land heating up more reversing the airflow. quickly than the sea, as both absorb solar radiation. This occurs because the sea absorbs ONSHORE BREEZE large quantities of heat energy with only a small rise in temperature, whereas the same amount of heat energy is cold air sinks air heats up likely to cause the land temperature to rise sharply (see p.31). cool air drawn and rises As the land warms up, it heats the air above it, causing the air to rise. over ocean seaward Cooler air then blows in from the sea to take its place. In the evening, and at night, the opposite effect occurs. At nightfall, the land quickly cools down, but the sea remains warm and continues to heat the air above it. As this warm air rises, it sucks the cooler air off the land, and so generates an offshore breeze. This is sometimes called a “land breeze.” OFFSHORE BREEZE
On warm coasts, there is often a noticeable drop in temperature from midday as a cool sea breeze blows in off the water. The breeze typically reverses in the evening and at night.
76
The regular movements of the tides are described here, as well as the way that disturbances spread out across the surface in the form of waves.
cold air sinks
Coastal Breezes
PATTERN OF WINDS
Year-round, the winds over most oceans are trades or westerlies. An exception is the northern Indian Ocean—this has a monsoon climate, in which a seasonal switch in wind direction occurs.
This chapter contains two sections. The introduction to ocean life covers the ecology and history of marine life and the way that marine organisms are classified. It is followed by a larger section, kingdoms of ocean life. This is divided into domains or kingdoms and, in the case of the plant and animal kingdoms, further divided into smaller groups. In each case, a general overview of the organisms that make up the group is followed by profiles of a selection of individual species. The section begins with the smallest forms of life, the bacteria and archaea, and ends with the animal kingdom.
KINGDOM Animalia
◀ CIRCULATION AND CLIMATE
southeasterly trade wind
Ocean winds are monitored by instruments called scatterometers, such as an instrument called ASCAT on the METOP-A satellite (right). A scatterometer is a radar device that can measure both wind speed and direction.
REEF FLAT OFF PANTAR ISLAND
This shallow reef area, featuring numerous species of stony coral and a starfish, is in east Nusa Tenggara.
DOMAIN Eucarya
LONG-HAUL SAILING
trade winds meet at Intertropical Convergence Zone
prevailing warm
southeasterly trade winds
rifts occur when fragments of crust move apart
solid inner core
air rises at equator
air descends at pole
coral continues to grow where waves bring food
Winds can blow with a consistent strength and direction over large areas of ocean. Consequently, on long-haul sailing trips, the same basic sail settings can often be used for days on end.
air descends in subtropical latitudes
southeasterly trade winds
southeasterlies
Earth had deep oceans from an early stage, with volcanoes and an increasing area of continental crust standing above the surface. The ocean became salty as weathering of surface rocks added minerals to the water.
liquid outer core
volcanic activity adds igneous rocks to surface above rising flows
Peridotite is the dominant rock type found in the mantle, consisting of silicates of magnesium, iron, and other metals. Sometimes it is brought to the surface when parts of the ocean floor are uplifted, as here in Newfoundland, Canada, or as fragments from volcanic activity.
KEY
southeasterly trade winds
rivers erode and transport sediment
INTRODUCTION
INTRODUCTION
Intertropical Convergence Zone
westerlies
sedimentary rocks
During the process of differentiation, volatile materials were expelled from Earth’s interior by volcanic activity. The lightest gases, such as hydrogen and helium, would quickly have been lost to space, leaving a stable atmosphere of nitrogen, carbon dioxide, and water vapor. Some of the water vapor would have condensed to form liquid water, and it seems there was a significant ocean earlier than 4 billion years ago. Some meteorites contain 15-20 percent ocean water from water and the early Earth is thought volcanic eruptions and comet to have had the same composition, impacts providing an ample source for the early ocean. More water arrived with impacting comets. It was in the ocean that free oxygen first appeared, with the arrival traces of early of photo- synthesizing life around 3.5 billion years ago. meteorite and comet
primitive oceanic crust
DISCOVERY
The Coriolis Effect
crust pulled apart by convective motion in mantle
43
BANDED IRON
Known as a banded-iron formation, this layered rock contains iron oxides that formed as the oxygen content of early oceans increased.
Hadley cell
Solar heating causes the air in Earth’s atmosphere to CIRCULATION CELLS The atmospheric cells cycle around the globe in three sets of giant loops, produce north–south called atmospheric cells. Hadley cells are produced by airflows. These are modified by Earth’s warm air rising near the equator, cooling in the upper spin, producing winds atmosphere, and descending to the surface around that blow diagonally. subtropical latitudes (30˚N and S). Then the air moves subtropical back toward the equator. Ferrel cells are produced by jet stream air rising around subpolar latitudes (60˚N and S), cooling and falling in polar-front jet stream the subtropics, and then moving toward the poles. Polar cells are caused by air descending at the poles and moving toward the equator.
Earth’s rotation
air rises in subpolar latitudes Ferrel cell
southwesterly wind
primitive continental crust thickens above sinking mantle flow, without mantle interference
direction of Earth’s spin
Atmospheric Cells
initial direction of air movement
polar cell
Moho
OCEAN-FLOOR STRUCTURE
Three layers of basalt in the crust (basaltic lava, dikes, and gabbro) are separated from the mantle by the Mohorovicˇic´ discontinuity (the Moho). The top layer of the upper mantle is fused to the base of the crust to form the rigid lithosphere, which makes up tectonic plates.The asthenosphere is the soft zone over which the plates of the lithosphere glide.
INTRODUCTION
CIRCULATION AND CLIMATE
ocean crust
lithosphere
magma rises to surface
basalt continuously intrudes from mantle
ZIRCON
Water and Atmosphere
Oceanic Crust
ocean surface
peridotite
INTRODUCTION
54
basalt sheets (dikes) sediment
gabbro
asthenosphere
central area filled by reef limestone
Ocean Life greenstone belts above rising mantle flow
zircon crystals, among the earliest continental crust materials
Continental Crust The continents include a wide range of rock THE OLDEST ROCKS types, including granitic igneous rocks, sedimentary These sedimentary rocks on Baffin Island rocks, and the metamorphic rocks formed by the lie on the Canadian alteration of both. They contain a lot of quartz, a Shield. The stable mineral absent in oceanic crust. The first continental continental shields rocks were the result of repeated melting, cooling, contain the world’s most ancient rocks, and remixing of oceanic crust, driven by volcanic activity above mantle convection cells, which were which are around 4 billion years old. much more numerous and vigorous than today’s. Each cycle left more of the heavier components in the upper mantle and concentrated more of the lighter components in the crust. The first microcontinents grew as lighter fragments of crust collided and fused. Thickening of the crust led to melting at its base and underplating with granitic igneous rocks. Weathering accelerated the process of continental rock formation, retaining the most resistant components, such as quartz, while washing solubles into the ocean. rift
volcanic island becomes submerged
These pages describe general types of environments. The example above is taken from the SHALLOW SEAS section.
THE ORIGIN OF OCEANS AND CONTINENTS DEVELOPMENT OF CONTINENTAL CRUST
Modification of the crust above rising mantle flows was delayed by the continuous intrusion of mantle basalt, resulting in the greenstone belts found today at the heart of each continental shield.
EARTH’S OCEANS FORMED MORE THAN 4 billion years ago, mainly from water vapor that condensed from its primitive atmosphere but also from water brought from space by comets. Initially, after acquiring a layered internal structure, the Earth had a uniform crust that was enriched in lighter elements and floated on an upper mantle made of denser materials. Later, the crust became differentiated into two types as continents began to form, made from rocks that were chemically distinct from those underlying the oceans.
basaltic lava
Bleaching refers to color loss in reef-building corals and occurs when the tiny organisms called zooxanthellae, which give corals their colors, are ejected from coral polyps or lose their pigment. In extreme cases, this can lead to the coral’s death.Various stresses can cause bleaching, including pollution, ocean temperature rise, and ocean acidification (see p.67). In recent decades, some mass bleaching events have affected reefs over wide areas.
coral continues to grow, forming barrier reef
INTRODUCTION
As well as describing the composition of the ocean floor, this section looks at the processes that shape it, tracing the origin of the oceans and their changing size and shape over geological time.
ATOLL
This satellite image of the Skagerrak (a strait linking the North and Baltic seas) shows a bloom of phytoplankton, visible as a turquoise discoloration in the water.
These tiny forms of planktonic organisms have cell walls made of silicate. They can only grow if there are sufficient amounts of silica present in the water.
OCEAN GEOLOGY
42
An atoll is shown here forming around a volcanic island. First, the island’s shore is colonized by corals forming a fringing reef (above). Over time, the island subsides, but coral growth continues, forming a barrier reef (above right). Finally, the island disappears, but the coral maintains growth, forming an atoll (right). Atolls can also form as a result of sea-level rise.
reef face
▲ EXPLANATORY PAGES
SILICEOUS DIATOMS
RIVER DISCHARGE
River discharge is a mechanism by which ions of sea salt and nutrients enter the oceans. Here, the Noosa River empties into the sea on the coast of Queensland, Australia.
CORAL BLEACHING
lagoon of shallow water
ATOLL FORMATION
PLANKTON BLOOM
dissolving of minerals from sea floor precipitation of minerals onto sea floor carbonates incorporated into seafloor sediments from animal shells
INTRODUCTION
INTRODUCTION
slow uplift of sedimentary rocks at continental margins, exposing salts, minerals, and ions at surface
–
sodium chloride crystal
Numerous substances present in small amounts in seawater are essential for marine organisms to grow. At the base of the oceanic food chain are phytoplankton—microscopic floating life-forms that obtain energy by photosynthesis. Phytoplankton need substances such as nitrates, iron, and phosphates in order to grow and multiply. If the supply of these nutrients dries up, their growth stops; conversely, blooms (rapid growth phases) occur if it increases. Although the sea receives some input of nutrients from sources such as rivers, the main supply comes from a continuous cycle within the ocean. As organisms die, they sink to the ocean floor, where their tissues decompose and release nutrients. Upwelling of seawater from the ocean floor (see p.60) recharges the surface waters with vital substances, where they are taken up by the phytoplankton, refueling the chain.
sodium ion (positive charge)
+
+
+
WATER AS A SOLVENT
The charge imbalance on its molecules makes water a good solvent. When dissolving and holding sodium chloride in solution, the positive ends of the molecules face the chloride ions and the negative ends face the sodium ions.
◀ OCEAN WATER
lagoon volcanic island
The body plan (far left) of gastropods (slugs and snails) features a head, large foot, and usually a spiral shell (left). In shelled forms, all the soft body parts can be withdrawn into the shell for protection, or to conserve moisture while uncovered by the outgoing tide.
shell mantle cavity
digestive system
siphon
muscular foot BIVALVE SHELL gill
adductor muscle
jaws feeding arm
radula
OCEAN LIFE
The Chemistry of Seawater
SOURCES, SINKS, AND EXCHANGES
33
OCEAN ENVIRONMENTS
THE CHEMISTRY OF SEAWATER
OCEAN ENVIRONMENTS
OCEAN WATER
OCEAN ENVIRONMENTS
HUMAN IMPACT
32
digestive system
eye arm
internal shell
siphon gill
mantle cavity
TYPE
Atol
330 (130 squar
AREA
CONDITION
recovering bleaching Central Sulu Sea, between t and northern Borneo
LOCATION
The Tubbataha Reefs lie arou atolls in the centre of the Sul are famous for the many larg (open ocean) marine animals to them – such as sharks, Ma turtles, and barracuda. The ste shelving reefs here are also ri smaller life, including many s of crustaceans, colourful nud (sea slugs), and more than 35 of stony and soft coral. In the early 1990s, the Tub Reefs were rated by scuba di among the top ten dive sites world. However, during the they suffered considerable da from destructive fishing pract the establishment of a seawee In this photograph of a steeply she reef slope, several species of soft are visible, together with a shoal o Longfin Bannerfish.
AREA
The conditions needed for the growth of warm-water coral reefs are found mainly within tropical areas of the Indian, Pacific, and Atlantic oceans. The reefs are chiefly in the western parts of these oceans, where the waters are warmer than in the eastern areas.
An atoll is a ring of coral reefs or coral islands enclosing a central lagoon. It may be elliptical or irregular in shape.
island subsides when volcano has become inactive
PACIFIC OCEAN WEST
Tubbataha Reefs
CORAL DROP-OFF
Distribution of Reefs
ATOLL
A barrier reef is separated from the coast by a lagoon. In this aerial view, the light blue area is the reef and the distant dark blue area is the lagoon.
sea level
Despite its name, the colour of this coral varies from violet through blue, turquoise, and green to yellow-brown. Its branching vertical plates can form massive colonies.
This group of branching hard corals is growing at a depth of about 16 ft (5 m) off the coast of eastern Indonesia. Individual stony corals can grow up to a few inches per year.
OPEN POLYPS
At the center of each polyp is an opening, the mouth, which leads to an internal gut. The tissue around the gut secretes limestone, which builds the reef.
WARM-WATER REEF AREAS
A fringing reef directly borders the shore of an island or large landmass, with no deep lagoon.
BLUE RIDGE CORAL
STONY CORAL
Types of Reefs Coral reefs fall into three main types: fringing reefs, barrier reefs, and atolls. The most common are fringing reefs. These occur adjacent to land, with little or no separation from the shore, and develop through upward growth of reef-forming corals on an area of continental shelf. Barrier reefs are broader and separated from land by a stretch of water, called a lagoon, that can be many miles wide and dozens of yards deep. Atolls are large, ring-shaped reefs, enclosing a central lagoon; most atolls are found well away from large landmasses, such as in the South Pacific. Parts of the reef structure in both atolls and barrier reefs often protrude above sea level as low-lying coral islands—these develop as wave action deposits coral fragments broken off from the reef itself. Two other types of reefs are patch reefs—small structures found within the lagoons of other reef types—and bank reefs, comprising various reef structures that have no obvious link to a coastline.
FRINGING REEF
species concentrated in a few square kilometres. The reef also contains the world’s largest colony of rare Blue Ridge Coral (Heliopora coerulea). For decades, environmentalists battled to save the reef from the building of a new airport for Ishigaki. A proposal to construct the airport on top of the reef was dropped, but concern remains now that it has been built on land, as discharge of excavated soil into the reef is likely to have an adverse effect.
CEPHALOPOD ANATOMY
Cephalopods have large eyes, in front of which there are a number of tentacles. The siphon functions in respiration and in rapid movement. Some forms have a flattened internal shell.
indicate an extremely high d marine life in this region. Fo a single large reef can contai than 1,200 species of fish (m in all the seas in Europe com and 500 different species of building coral. Common ani include Eagle Rays, Manta R
7
Atlas of the Oceans PACIFIC OCEAN SOUTHWEST
Great Barrier Reef TYPE
map shows location and, in most profiles, geographical extent of feature
Barrier reef table of summary information (categories vary between sections)
14,300 square miles (37,000 square km) AREA
CONDITION Damaged by Crown-of-thorns Starfish; coral bleaching
Parallel to Queensland coast, northeastern Australia
LOCATION
Parallel to Queensland coast, northeastern Australia
LOCATION
A
HABITAT
Middle and lower rocky shores
Caribbean, Bahamas, Florida
DISTRIBUTION
Northwestern and northeastern
Atlantic
OCEAN LIFE
One of the most common rocky shore gastropods, the dog whelk has a thick, heavy, sharply pointed spiral shell. The shell’s exact shape depends on its exposure to wave action, and its color depends on diet. Dog whelks are voracious predators, feeding mainly on barnacles and mussels. Once the prey has been located, the whelk uses its radula to bore a hole in the shell of its prey before sucking out the flesh.
Sense Organs
PIGMENTED SKIN CELLS HELP CUTTLEFISH TO CHANGE COLOR
When the cuttlefish passes over a darker background, it disperses the colored pigments throughout each of its chromatophores, and the animal darkens.
2
MOLLUSCAN BEAUTY
Displaying fabulous warning colors, this nudibranch is a shell-less example of the many thousands of marine species of gastropods (slugs and snails).
K
il ur
re eT
nc
33m (108ft)
B
1,857m (6,093ft)
Bowers Attu Attu Near Basin Island Islands Agattu Island
e
u
DISTRIBUTION
HIDING FROM VIEW There are times when the Venus comb buries itself just below the surface of the sea floor, displacing the sand with movements of its muscular foot. However, it leaves the opening of its tubular inhalant siphon above the sand’s surface so that it can draw water into its mantle cavity to obtain oxygen and to “taste” the water for the presence of prey.
CLASS GASTROPODA
Giant Triton Charonia tritonis LENGTH
Up to 16 in (40 cm) HABITAT
Coral reefs, mostly in subtidal zones
DISTRIBUTION
Indian Ocean, western and central
Pacific
CLASS GASTROPODA
HALF BURIED
The spines of this Venus comb can be seen sticking out of the sand. The siphon is visible to the right of the picture.
Common Periwinkle Littorina littorea CLASS GASTROPODA
Flamingo Tongue LENGTH
1–11/2 in (3–4 cm) HABITAT
Western Atlantic, from North Carolina to Brazil; Gulf of Mexico, Caribbean Sea
277
The off-white shell of the flamingo tongue cowrie is usually almost completely hidden by the two fleshy, leopard-spotted extensions of its
body’s outer casing, or mantle. When threatened, however, its distinctive coloration quickly disappears as it withdraws all its soft body parts into its shell for protection. This snail feeds almost exclusively on gorgonian corals, which dominate Caribbean reef communities. Although these corals release chemical defenses to repulse predators, the flamingo tongue cowrie is apparently able to degrade these bioactive compounds and eat the corals without coming to any harm. After mating, the female strips part of a soft coral branch and deposits the egg capsules on it. Each capsule contains a single egg that will hatch into a free-swimming planktonic larva.
LENGTH
Up to 1 in (3 cm) HABITAT
Upper shore to sublittoral rocky shores, mud flats, estuaries Coastal waters of northwest Europe; introduced to North America
DISTRIBUTION
This gastropod is one of the very few animals that eats the crown-of-thorns starfish, itself a voracious predator and destroyer of coral reefs. The giant triton is an active hunter that will chase prey, such as starfish, mollusks, and sea stars, once it has detected them. It uses its muscular single foot to hold its victim down while it cuts through any protective covering using its serrated, tonguelike radula; it then releases paralyzing saliva into the body before eating the subdued prey. directly into the water during the spring tides. The eggs hatch into free-swimming larvae that float in the plankton for up to six weeks. After settling and metamorphosing into the adult form, it takes a further two to three years for the adult to fully mature. It feeds mainly on algae, which it rasps from the rocks. In the 19th century, the common periwinkle was accidentally introduced into North America, where its selective grazing of fast-growing algal species has considerably affected the ecology of some rocky shores.
The common periwinkle has a black to dark gray, sharply conical shell and slightly flattened tentacles, which in juveniles also have conspicuous black banding. The sexes are separate and fertilization occurs internally. Females release egg capsules, containing two or three eggs,
◀ SPECIES PROFILES
GRAFTING OYSTERS
All species profiles contain a text description and, in most cases, a color photograph and distribution map.
Pearls form in oysters when a grain of sand or other irritant lodges in their shells. The oyster coats the grain with a substance called nacre, forming a pearl. Today many pearls are cultured artificially: the shell is opened just enough to introduce an irritant into the mantle cavity.
CLASS GASTROPODA
Flamingo Tongue
Movement Mollusks move in many different ways. Most gastropods glide across surfaces using their mucus-lubricated foot. Exceptions include the sea butterfly, which has a modified foot with finlike extensions for swimming. Some bivalves, such as scallops, also swim, producing jerky movements by clapping the two halves of their shell together. Other bivalves burrow by probing with their foot and then pulling themselves downward by muscular action. Cephalopods are efficient swimmers; some have fins on the sides of their bodies that let them hover in the water, and they can accelerate rapidly by squirting water out through their siphons.
Cyphoma gibbosum LENGTH
1–11/2 in (3–4 cm)
name of group to which species belongs common name of species is followed by scientific name
HABITAT
Coral reefs at about 50 ft (15 m)
AIDED BY MUCUS
Muscular contractions ripple through the fleshy foot of this marine snail. It secretes a lubricating mucus that helps it to move on rough surfaces.
Respiration Most mollusks obtain oxygen from water using gills, called ctenidia, which are situated in the mantle cavity. These are delicate structures with an extensive capillary network and a large surface area for gaseous exchange. In species that are always submerged, water can continually be drawn in and over the gills. Those living in the intertidal zone are exposed to the air for short periods and must keep their gills moist. At low tide, bivalves clamp shut and some gastropods close their shell with a “door” (called an operculum) to retain moisture. Pulmonate snails have a simple lung formed from the mantle cavity instead of ctenidia and are mostly terrestrial but others live on the seashore and can absorb oxygen through their skin when immersed.The respiratory pigment in most molluscan blood is a copper compound called hemocyanin. It is not as efficient at taking up oxygen as external gills hemoglobin and gives mollusks’ (ctenidia) blood a blue color.
Nudibranchs (sea slugs) have feathery external gills toward the rear of their bodies. The warning coloration of this species includes the bright orange gills.
DISTRIBUTION Western Atlantic, from North Carolina to Brazil; Gulf of Mexico, Caribbean Sea
OCEAN LIFE
COLOR CODING
en
Riv er
Yuk on
Riv kw im
ko
Ra t I sla nd s
Kiska Island Amchitka Island
ch
20m (66ft)
Inlet
K
Cook
rai t
a Pe
ni
ns
f ko
St
Tanaga A Island
ds leutian Islan Atka Island
Atka
n
Ale
Isla Fox
E
Po r B a tl oc nk k
Kodiak
Kodiak Island
i
Tr
en
ch
Gulf of Alaska
Patton Seamount Patt on
Seamou nts
Murray Seamount Gilbert Seamount
G i lb er
Parker Seamount
5,267m (17,281ft)
160˚W
F
ut
an
G
2
sea depth maximum depth on map
Chichagof Admiralty Island Island 295m A Sitka (968ft) 3,640m Baranof (11,943ft) Island Ketchikan of Alask a Seamount Pr ovinc Prince of e Gulf Pratt Wales Quinn Seamount Prince Rupert Island n Giacomini Seamount Durgin o ce Seamount Dixtran Seamount K Cape En o Surveyor Seamdiak Knox Seamount ount Dickins s Welker Seamount
Shumagin Islands
so Dutch Davidnk Umnak Harbor Ba Island Unalaska nds Island
Juneau
Glacier Bay
Port Moller Unimak Island False Pass
7,314m 170˚W (23,997ft)
D
Bristol Bay 62m (203ft)
ul
Cape Constantine
Umnak Be Plateau ring Cany on
Andreano f Islands
180˚
C
Cape Newenham
Pribilof Islands
6,102m (20,021ft)
Shuyak Island
li
Bering Sea
11m (36ft) Bowers Bank
Homer
seamount
Yakutat
Prince Cape William Saint Elias Sound
tectonic plate boundary
Gibson Seamount
150˚W
H
Seamount
Denson Seamount
Alaska Plain
White Marsh Seamount
t Se a
mo
Schoppe
un
I
Ridge
Peters
ts
766m (2,513ft)
e en
Ch a
rlo t Bowie te Seamount Oshawa Seamount
Miller Seamount
Ridge
140˚W
Isla n
3
213m
t (699ft) Cape St.James QueenCharlotte Sound Cape Scott
ds
Explorer Seamount
130˚W
J
K
50˚N
rait of
Vancouve r
Vancouver
G Island eorgia
Victoria Strait of Seattle Juan de Fuca
4
UNITED STATES OF AMERICA
Cascadia Basin L
John Sparks is a curator in the Department of Ichthyology at the American Museum of Natural History and an adjunct professor at Columbia University in New York City. The revised edition was prepared with the help of Mark Siddall, a curator in the Division of Invertebrate Zoology and a professor at the Richard Gilder Graduate School at the American Museum of Natural History.
Indian Ocean, western Pacific
One of the largest cowrie species, the tiger cowrie has a shiny, smooth, domed shell with a long, narrow aperture, and is variously mottled in black, brown, cream, and orange. The cowrie’s mantle (its body’s outer, enclosing layer) can extend to cover parts of the exterior of the shell. These extensions have numerous projections, or papillae, whose exact function is unknown, but which may increase the surface area for oxygen absorption or provide camouflage of some sort. Tiger cowries are nocturnal creatures, hiding in crevices among the coral during the day and emerging at night to graze on algae. The sexes are separate and fertilization occurs internally. Females exhibit some parental care in that they protect their egg capsules by covering them with their muscular foot until they hatch into larvae, which then enter the plankton to mature.
DISTRIBUTION
Swimming backward reduces drag from the tentacles. The siphon, used for jet propulsion, is clearly visible in this Humboldt squid.
Tr
20m (66ft)
ai la en insu Seward n Pe
CONSULTANTS
Low tide to 100 ft (30 m) on coral reefs and flats
Cyphoma gibbosum
siphon
Takoma Reef
an
170˚E
HABITAT
Coral reefs at about 50 ft (15 m)
REDUCING DRAG
ti
16,400 ft (5,000 m)
it Kuskokwim Bay
Nunivak Island
4,024m (13,203ft) Ulm Bowers Seamount Plateau
A
Northwest Pacific Basin
160˚E
Aleutian Basin
Komandorskiye Ostrova
Ostrov Mednyy
6,088m (19,975ft)
4
Pervenets Canyon
land
Cordova
S he
h
LENGTH
SEEDING AN OYSTER
The giant cuttlefish’s color change is due to skin cells called chromatophores. It is pale when pigment is confined to a small area of each cell.
˚N
Mednyy Seamount
9,800 ft (3,000 m)
C A N A D A
Anchorage
Et
Up to 6 in (15 cm)
The best-shaped artificial pearls are produced by “seeding” oysters with a tiny pearl bead and a piece of mantle tissue from another mollusk .
1
50
635m (2,083ft)
er
s Ku
Hooper Bay Saint Matthew Island
285
HUMAN IMPACT
Touch, smell, taste, and vision are well developed in many mollusks. The nervous system has several paired bundles of nervous tissue (ganglia), some of which operate the foot, and interpret sensory information such as light intensity. Photoreceptors range from the simple eyes (ocelli) seen along the edges of the mantle or on bivalve siphons, to the sophisticated image-forming eyes of cephalopods. Cephalopods are also capable of rapidly changing their color.
cha e
Mys Kam rr ac 7,864m Shipunskiy Te (25,802ft) 3
29m (95ft)
Mys Olyutorskiy
CLASS GASTROPODA
three to five clearly visible holes in the shell, through which water flows for respiration.These are filled and replaced with new holes as the abalone increases in size. Sea otters are one of the red abalone’s main predators, along with human divers.
MOLLUSKS
Kamchatskiy Zaliv Ostrov Beringa Petropavlovsk- Kronotskiy Kamchatskiy Zaliv tka
12m (39ft)
Olyutorskiy Zaliv
Tiger Cowrie
OCEAN LIFE
The small, rounded, smooth, blackand-white striped shell of the zebra nerite is typical of the species, but in examples from Florida the shell is sometimes more mottled or speckled with black. These gastropods are most active during the day, when they feed on microorganisms such as diatoms and cyanobacteria, but if they become too hot or they are exposed at low tide, they cluster together, withdraw into their shells, and become inactive. This may be a mechanism for preventing excessive water loss. Unusually for gastropods, there are separate males and females of zebra nerites and fertilization of the eggs occurs internally. The males use their penis to deposit sperm into a special storage organ inside the female. Later, she lays a series of small white eggs that hatch into planktonic larvae.
li v y Za i ns k Ostrov Karaginskiy
Kamchatka Basin Mys Sivuchiy
shaded area of map shows known natural range of species
table of summary information (varies between categories) all distribution maps are accompanied by a written summary of the range of the species
CONTRIBUTORS Richard Beatty Glossary Kim Bryan Introduction to Ocean Life, Bacteria and Archaea, Protists, Fungi, Mollusks, Arthropods, Red Crab Migration David Burnie Animal Life, Reptiles, Birds, Mammals Robert Dinwiddie Ocean Water, Circulation and Climate, Tides and Waves, Coasts and the Seashore, Shallow Seas, Polar Oceans, Ocean Yacht Racing, Shutting Down the Atlantic Conveyor, Hurricane Katrina, Global Warming and Sea-level Rise, Coastal Defenses, The Titanic Disaster Frances Dipper Introduction to Ocean Life, Sponges, Cnidarians, Segmented Worms, Flatworms, Ribbon Worms, Bryozoans, Echinoderms, Small Bottom-living Phyla, Planktonic Phyla, Tunicates and Lancelets, Jawless Fishes, Cartilaginous Fishes, Bony Fishes Philip Eales Ocean Geology, Atlas of the Oceans, Oceanography from Space, The Indian Ocean Tsunami, Ice-shelf Breakup Monty Halls Diving Tourism Sue Scott Shallow Seas, Red and Brown Seaweeds, Plant Life, Green Seaweeds, Green Algae, Mosses, Flowering Plants, Fishing Michael Scott The Open Ocean and Ocean Floor, Exploration with Submersibles, Cold Water Reefs, Biodiversity Hotspots, Whale Migration, Wind Farming in the Baltic The revised edition was prepared with the help of David Burnie (Ocean Life), Robert Dinwiddie (Introduction and Ocean Environments), Frances Dipper (Ocean Life), and Philip Eales (Atlas of the Oceans)
AT L A S O F T H E O C E A N S
LENGTH
Up to 2 1/2 in (6 cm)
HABITAT
rag
St
DISTRIBUTION
LENGTH
Up to 1/2 in (1 cm) Rocky tide pools
Pacific
The tropical carnivorous snail known as the Venus comb has a unique and spectacular shell. There are rows of long, thin spines along its longitudinal ridges, which continue onto the narrow, rodlike, and very elongated siphon canal. The exact function of these spines is unknown, but they are thought to be either for protection or to prevent the snail from sinking into the soft substrate on which it lives. Its body is tall and columnar so that it can lift its cumbersome shell above the sediment to move in search of food.
Ka
tk a
Qu
Nucella lapillus
a
cha
1,600 ft (500 m)
6,500 ft (2,000 m)
go
Dog Whelk
Puperita pupa
Eastern Indian Ocean and western
s ul
Ka m
1
800 ft (250 m)
500 miles
3,300 ft (1,000 m)
ai Str cate He
Zebra Nerite
DISTRIBUTION
nin
sea level
400
˚N 60
el a
CLASS GASTROPODA
e aP
Ust’-Kamchatsk
110˚W
500 km
300
ip
Tropical warm waters to 650 ft (200 m) CLASS GASTROPODA
at k
400
200
r ch
HABITAT
ch
Norton Sound
300
100
A
LENGTH
Up to 3 in (8 cm)
2
Saint Lawrence Island
Mys Navarin
Nome Norton Plain
Khatyrka Ossora
Cypraea tigris
HABITAT
Venus Comb
L
KEY 200
100
0
er
Easily distinguished from most other gastropods by the conical shape of its spiral shell, the top shell moves slowly over reef flats and coral rubble, feeding on algae. Demand for its flesh and pretty shell has led to declining numbers, especially in the Philippines, due to unregulated harvesting. It has, however, been successfully introduced elsewhere in the Indo-Pacific, such as French Polynesia and the Cook Islands, from where some original sites are being restocked.
LENGTH
Murex pecten
K 120˚W
SCALE
nd
Reef fish, including Longfin Bannerfish, Milletseed Butterflyfish, and Bluestripe Snappers, swim around a table coral.
6–8 in (15–20 cm)
CLASS GASTROPODA
RETURNING HOME
J 130˚W
140˚W
UNITED STATES OF A MERICA
a
FRENCH FRIGATE SHOALS
Rocks from low tide mark to 100 ft (30 m)
Limpets gradually grind a “scar” into their anchor spot on the rock, to aid their grip and help retain water. A mucus trail leads them back to the spot.
I Arctic Circle
0
Chirikof Basin
Mys Chukotskiy
Sea of Okhotsk
In most cases, explanatory pages are followed by profiles of actual features. For example, the profiles shown here describe coral reefs from around the world. Most profiles are illustrated with color photographs.
Haliotis rufescens
The largest of the abalone species, the red abalone is so called because of the brick-red color of its thick, roughly oval shell. There is an arc of
Fuca Plate west of Vancouver Island. The seamounts were created above the hotspot over the last 30 million years, then carried northwest by seafloor spreading. Since 1977, oil has been shipped through ports on the south coast of Alaska. In 1989, Prince William Sound was the site of one of the worst maritime environmental disasters, when the tanker Exxon Valdez ran aground, releasing about 30 million gallons (114 million liters) of crude oil.
were born. The floor of the Gulf of Alaska is peppered with seamounts. There are two main chains: the Patton and Gilbert seamounts, and the Kodiak Seamounts, both running away from the Alaska Peninsula. Their origin is the Cobb Hotspot, situated beneath the spreading center of the Juan de
H 150˚W
160˚W
sk
A wide fringing reef almost completely surrounds the shoreline of mountainous Moorea, part of which is visible in this view.
Red Abalone
East Pacific coasts from southern Oregon, US to Baja California, Mexico
G Good Bayhop
Seward Peninsula
la
MOOREA
CLASS GASTROPODA
DISTRIBUTION
ALASKAN FJORD
The valleys and fjords of the Alexander Archipelago testify to extensive erosion by glaciers during the last ice age.
xa
Eastern Indian Ocean, western and southern Pacific
DISTRIBUTION
The Cascadia Basin is the last remnant of the original eastern Pacific oceanic plate, the Farallon Plate, which has been almost entirely subducted beneath North America. The Cascade Range of volcanoes in Oregon and Washington State, including Mount St. Helens, are a product of this subduction. Mount St. Helens erupted in a catastrophic explosion in 1982, killing 57 people, and still shows signs of activity. Earthquakes and associated tsunamis are also a risk in the area, although the last major earthquake is thought to have been in 1700. The underlying ocean crust appears to be split into three small plates. The largest is the Juan de Fuca Plate, named after a Greek sea captain who explored the area for Spain in 1592. The Explorer Plate lies to the north and the Gorda Plate to the south.
le
The common limpet’s muscular foot, seen here from below, holds it firmly to its rock, regardless of the strength of the waves.
taller shells allow for better water retention during periods of exposure. Limpets travel slowly during low tide, covering up to 24 in (60 cm) using contractions of their single foot. They graze on algae from rocks using a radula (a rasplike structure), which has teeth reinforced with iron minerals.
Pacific Ocean; Columbia, Fraser rivers
INFLOWS
A counterclockwise subpolar gyre extends across the north Pacific and into the Gulf of Alaska, fed by the warm waters of the northern Kuroshio Extension, the extension of the Kuroshio Current. The surface waters are cooled and become less saline due to precipitation as they cross the ocean. Many of the storms that lash the west coast of Canada originate in the Gulf of Alaska. The circulation is completed as the Alaska Current and the Aleutian Current return west along the Alaskan coast and south of the Aleutian Islands. The gulf ’s waters are very productive, providing feeding grounds for many species of fish. Pacific salmon spend up to five years at sea, much of it in the gulf and adjacent seas, before returning to spawn in the Asian and North American rivers where they
a Str
Abundant on rocks from the high to the low water mark, the common limpet is superbly adapted to shore life. A conical shell protects it from predators and the elements. Limpets living at the low water mark are buffeted by the waves and so require smaller, flatter shells than those living at the high water mark, where wider,
9,600 ft (2,930 m)
INFLOWS
n oli
Northeastern Atlantic from Arctic Circle to Portugal
DISTRIBUTION
66,000 sq. miles (170,000 sq. km)
MAXIMUM DEPTH
A
◀ FEATURE PROFILES
Gulf of Anadyr
idge rs R we Bo
Intertidal and shallow subtidal areas, reef flats to 23 ft (7 m)
R US S I AN F EDER ATI ON
ti
HABITAT
conical shell
MUSCULAR FOOT
F 170˚W
Chukchi Sea Chukotskiy Poluostrov
Anadyr’
se l A n Ri a
6 in (16 cm)
HABITAT
E Arctic Circle
180˚
dy r’
eu
LENGTH
DIAMETER
21/2 in (6 cm) Rocks on high shore to sublittoral zone
D 170˚E
160˚E
150˚E
Stra it
˚N
ing
60
Al
CLASS GASTROPODA
Top Shell Tectus niloticus
THE BERING STRAIT
This satellite image shows ice from the Chukchi Sea streaming south through the Bering Strait.
C
1
MOLLUSKS orange foot with greenish tint
CLASS GASTROPODA
Common Limpet
AREA
16,400 ft (5,000 m)
Susitna, Copper rivers; icebergs from numerous glaciers
B er
The Society Islands comprise a chain of volcanic and coral islands in the South Pacific, including islands with barrier reefs (such as Rai’atea), islands with both fringing and barrier reefs (such as Tahiti), and atolls or nearatolls (such as Maupihaa and Maupiti). The reefs’ biological diversity is moderate compared with the reefs of Southeast Asia, although more than 160 coral species, 800 species of reef fish, 1,000 species of mollusc, and 30 species of echinoderm have been
ANIMAL LIFE
Patella vulgata
B
e
North-central Pacific
The Hawaiian Archipelago consists of the exposed peaks of a huge undersea mountain range. These mountains have formed over tens of millions of years as the Pacific Plate moves
PACIFIC OCEAN L4
Cascadia Basin
600,000 sq. miles (1.5 million sq. km)
CONDITION
French Polynesia, northeast of New Zealand, south-central Pacific
R is
LOCATION
AREA
MAXIMUM DEPTH
LOCATION
ev
Coral disease outbreaks reported
CONDITION
Good, but significant local damage
where the contact is between ocean crust and continental crust. The largest volcanic event of the 20th century was the eruption of Mount Katmai on the Alaskan Peninsula in 1912. This boundary can also produce powerful earthquakes such as the event that destroyed part of Anchorage in 1964.
ch
1,180 square km (450 square miles)
AREA
1,500 square km (600 square miles)
AREA
SEALS IN THE ALEUTIAN ISLANDS
ru
Fringing reefs, atolls, submerged reefs
TYPE
3 in (8 cm) per year
Ob
Hawaiian Archipelago
northwest over a hotspot in the Earth’s mantle. Coral reefs fringe some coastal areas of the younger, substantial islands at the southeastern end of the chain, such as Oahu and Molokai. To the northwest, located on the submerged summits of older, sunken islands, are several near-atolls (such as the French Frigate Shoals) and atolls (such as Midway Atoll). These reefs are highly isolated from all other coral reefs in the world, and although their overall biological diversity is relatively low, many new species have evolved on them. The more remote reefs are healthy, but in 2013, a serious coral disease was reported affecting reefs on Oahu and Kauai.
26,600 ft (8,100 m)
The Bering Sea is bounded to the south by the Aleutian Islands. On the Pacific side of the islands lies the Aleutian Trench, marking where the Pacific Plate is plunging beneath the North American Plate. It is this subduction zone that gives rise to the volcanic arc of islands, the most northerly link in the Pacific Ring of Fire. The trench continues to the east,
OCEAN ENVIRONMENTS
PACIFIC OCEAN CENTRAL
2,000 miles (3,200 km)
RATE OF CLOSURE
Amuk ta P a ss
As with many Pacific atolls, the rim of Majuro Atoll consists partly of shallow submerged reef and partly of small, low-lying islands.
LENGTH
Pacific Ocean; Yukon, Anadyr’ rivers
The Bering Sea is named after a Danish navigator in the Russian Navy, who explored the area in 1741. It lies between mainland Asia and North America, and is bounded by the Aleutian Islands to the south and linked to the Arctic Ocean in the north by the narrow Bering Strait. There is a flow of cold Arctic water south through this strait, feeding a counterclockwise circulation. The main freshwater input is the Yukon River, which has deposited an extensive delta at its mouth. The Bering Sea is one of the world’s richest fisheries, helping Alaska account
PACIFIC OCEAN I3
Gulf of Alaska
e
MAJURO ATOLL
Aleutian Trench
20,021 ft (6,102 m)
aP ass
Generally good; some episodes of coral bleaching
CONDITION
Fringing reefs, barrier reefs, atolls
TYPE
INFLOWS
PACIFIC OCEAN
MAXIMUM DEPTH
890,000 square miles (2.3 million square km)
Am chi tk
6,200 square km (2,400 square miles)
AREA
Micronesia, southwest of Hawaii, western Pacific
LOCATION
Society Islands
AREA
MAXIMUM DEPTH
THE COLD, STORMY SUBPOLAR SEAS of the North Pacific are highly productive, supporting a rich fishery. Geologically, the area is dominated by a subduction zone, and the area’s volcanoes and earthquakes pose an ever-present danger.
recorded. The reefs’ health is generally good, but some reefs around the busy holiday destination islands of Tahiti, Moorea, and Bora-Bora have been severely affected by construction, sewage, and sediment run-off.
for about half of the total US fish and shellfish catch. Harbor seals and gray whales also take advantage of these productive waters. In contrast to the deep ocean basin beneath the southwestern half of the sea, the broad continental shelf in the northwest is very shallow. Much of this area formed a land bridge during the last ice age, when sea levels were up to 390 ft (120 m) lower than they are today. This route was ice-free for extended periods, allowing several species, including humans, to migrate from Asia to North America on foot for the first time.
PACIFIC OCEAN D3
Bering Sea
An a
Humphead Parrotfish, and various species of octopuses and nudibranchs (sea slugs). Major threats to the reefs in Nusa Tenggara include pollution from land-based sources, removal of fish for the aquarium trade, and reef destruction by blast fishing. Coral bleaching affected some reefs in 2010.
Atolls
PACIFIC OCEAN SOUTHWEST
459
The Bering Sea And Gulf of Alaska
ge
TYPE
The Marshall Islands consist of 29 coral atolls and five small islands in the western Pacific. The atolls lie on top of ancient volcanic peaks that are thought to have erupted from the ocean floor 50-60 million years ago. They include Kwajalein, the largest atoll in the Pacific at 2,500 square km (1,000 square miles), and Bikini and Enewetak atolls, which were used by the USA for testing nuclear weapons between 1946 and 1962. Human pressures on these two remote, evacuated atolls have been minimal during the past 50 years, and marine life around them now thrives; for example, 250 species of coral and up to 1,000 species of fish have been recorded at Bikini.
THE PACIFIC OCEAN
458
One of the tiniest residents of the Great Barrier Reef, at just 7–8mm (less than 1⁄3in) long from snout to tail, is the Stout Infantfish. When discovered in 2004, the Infantfish was declared to be the world’s smallest vertebrate species. That title has since been claimed first by a slightly smaller species of Indonesian cyprinid fish, and more recently by a tiny species of frog, about 7mm (1⁄4in) long, found in Papua New Guinea.
Rid
Marshall Islands
However, a study published in 2012 reported that the reef has lost more than half its coral cover since 1985. The factors causing this damage include pollution, tropical cyclones, raised water temperature causing mass coral bleaching, population outbreaks of the Crown-of-thorns Starfish, overfishing, and shipping accidents.
THE WORLD’S SMALLEST VERTEBRATE?
ov
PACIFIC OCEAN SOUTHWEST
In this view of a central area of the reef, a deep, meandering channel separates two reef platforms. The region’s high tidal range drives strong currents through such channels.
sh
lving coral f
REEF CHANNEL
ir
Australia’s Great Barrier Reef, which stretches 2,010km (1,250 miles), is the world’s largest coral reef system. Often described as the largest structure ever made by living organisms, it in fact consists of some 3,000 individual reefs and small coral islands. Its outer edge ranges from 30 to 250km (18 to 155 miles) from the mainland, and its biological diversity is high. The reef contains about 350 species of stony coral and many of soft coral. Its 1,500 species of fish range from 45 species of butterflyfish, to several shark species, including silvertip, hammerhead, and whale sharks. The reef is also home to 500 species of algae, 20 species of sea snake, and 4,000 species of mollusc.
284
all maps are accompanied by a written description of location
Damaged by tropical storms, pollution, and an unbalanced ecosystem
CONDITION
bbataha vers in the 1980s mage tices and ed farm.
diversity of r example, n more more than mbined), reefimals here Rays,
Barrier reef
37,000 square km (14,300 square miles)
AREA
Sh
TYPE
Em pe r or Se a mou nts
he Philippines
und two u Sea and e pelagic attracted nta Rays, eeply ch in pecies ibranchs 0 species
Great Barrier Reef
m
Good; from coral n 2010
161
PACIFIC OCEAN SOUTHWEST
Ka
s
square km e miles)
In 1988, the Philippines government intervened, declaring the area a National Marine Park, and since 1993 it has also been a UNESCO World Heritage Site. The condition of the Tubbataha reefs has much improved, due to the enforcement of measures such as a prohibition on fishing and a ban on boats anchoring on the reefs (visiting craft must use mooring buoys). A setback occurred in January 2013 when a US Navy minesweeper ran aground on the reef, damaging over 2,000 square m (21,500 square ft).
The final chapter of the book is an atlas of the world’s oceans. It includes maps of the five major oceans. The pages that immediately follow each whole-ocean map contain more detailed maps of selected regions of that ocean. All the maps have been produced using data collected from a combination of satellite- and ship-borne instruments. They are labeled to show the names of the seas, undersea features (such as ridges, trenches, and seamounts), and prominent coastal features. They also show ocean depths and ▼ REGIONAL MAP the boundaries between tectonic As well as maps, these pages also include plates. Features identified on the profiles of individual seas or undersea maps have been included in the features. The example shown here is from the section on the Pacific Ocean. index at the end of the book.
AT L A S O F T H E O C E A N S
name of ocean in which feature is found
compass direction indicates position within ocean
FOREWORD
W
e should call our planet Ocean. A small orb floating in the endless darkness of space, it is a beacon of life in the otherwise forbidding cold of the endless universe. Against all odds, it is also the Petri dish from which all life known to us springs. Without water, our planet would be just one of billions of lifeless rocks floating endlessly in the vastness of the inky-black void. Even statisticians revel in the improbability that it exists at all, with such a rich abundance of life, much less that we as a species survive on its surface. Yet, despite the maze of improbability, we have somehow found our way to where we are today. Humans were enchanted by the sea even before the Greek poet Homer wrote his epic tale of ocean adventure, the Odyssey. It is this fascination that has driven us to delve into this foreign realm in search of answers, but the sea has always been reluctant to give up its secrets easily. Even with the monumental achievements of past explorers, scientists, and oceanographers, we have barely ventured through its surface. It is estimated that over 90 percent of the world’s biodiversity resides in its oceans. From the heartbeat-like pulsing of the jellyfish to the life-and-death battle between an octopus and a mantis shrimp, discoveries await us at every turn. And for every mystery solved, a dozen more present themselves. These are certainly exciting times as we dive into the planet’s final frontier. Aided by new technology, we can now explore beyond the two percent or so of the oceans that previous generations observed. But even with the advent of modern technology, it will take several more generations to achieve a knowledge base similar to the one we have about the land. No matter how remote we feel we are from the oceans, every act each one of us takes in our everyday lives affects our planet’s water cycle and in return affects us. All the water that falls on land, from the highest peaks to the flattest plains, ends up draining into the oceans. And although this has happened for countless millions of years, the growing ecological footprint of our species in the last century has affected the cycle in profound ways. From fertilizer overuse in landlocked areas, which creates life-choking algal blooms thousand of miles away, to everyday plastic items washing up in even the most remote areas of the globe, our actions affect the health of this, our sole life-support system. This statement is not here to make us feel that we are doomed by our actions, but rather to illustrate that through improved knowledge of the ocean system and its inhabitants, we can become impassioned to work toward curing our planet’s faltering health. By taking simple steps, such as paying a little more attention to our daily routines, each one of us can have a significant positive impact on the future of our planet and on the world our children will inherit. In short, it would be much healthier for us to learn to dance nature’s waltz than to try to change the music.
MOVING EN MASSE
The fast, coordinated movement of a shoal of fish is one of the most spectacular sights in the oceans. These blackfin barracuda have formed a spiraling shoal in water around the Solomon Islands. Such shoals are often found in the same place several months, or even years, apart.
Fabien Cousteau
GOLDEN JELLYFISH
Drawn to the light, Jellyfish in the genus Mastigias follow the sunlight around their brackish lake home in Palau. Stained golden-brown by algae in their tissues, they carry their photosynthetic passengers to the best-lit areas. In return, the algae provide them synthesized food. Isolated from the sea, these jellyfish are found nowhere else in the world.
NORTHERN EXPOSURE
The world’s shorelines can be inhospitable places to live. Common murres nest in colonies, favoring rocky cliffs. These birds are clinging to a rock off the Scottish coast, while being battered by a fierce gale. During the storm, many of the birds were swept off the rock and some of their eggs were washed into the sea. PROTECTION OF THE YOUNG
The packhorse lobster inhabits the continental shelves off Australia and New Zealand. The female shown here is carrying eggs under her abdomen. Up to two million eggs at a time can be stored in this way. Despite producing eggs in such prodigious numbers, these lobsters are threatened by overfishing and catches are now restricted.
FILTER-FEEDING WITH FEATHERS
Instead of moving around in search of food, many marine animals spend their lives fixed to the sea bed, collecting food as it drifts past. These feather-like funnels are actually part of the body of a worm. The worm beats tiny, hair-like structures to set up a current through the funnels and then traps food from the moving water.
NEW COASTLINE
The shape of a coastline is determined by a balance of forces. The coastlines of the Galápagos Islands in the eastern Pacific are relatively new, having formed when the islands were created by volcanic eruptions. The lava seen here solidified about 100 years ago, but more recent eruptions have occurred on some of the group’s younger islands. THE FALL OF THE APOSTLES
On this part of the southern Australian coast, marine erosion is the dominant force. A line of limestone cliffs is slowly being worn back by the sea, leaving behind isolated stacks of rock. The stacks are collectively known as the Twelve Apostles, although when they were named there were only nine of them and there are now just eight.
LIVING ON THE BOTTOM
The flattened body of a ray is an adaptation for life on the bottom of the sea. Most rays feed on animals on the seabed, and so their mouths are on the undersides of their bodies. They have flat teeth, which they use to grasp and then grind food. The features that resemble eyes are actually the ray’s nostrils.
AVOIDING A STING
Clownfish have a remarkable relationship with some anemones, feeding and sleeping among them. The anemones’ stinging tentacles repel all other fish, but the clownfish avoid triggering the firing of the anemones’ stinging cells with an undulating swimming action and by secreting chemicals that suppress the firing process. LEARNING TO SWIM
Being able to swim is a useful skill for polar bears, which for part of the year track their prey across shifting sea ice in the Arctic Ocean and are sometimes seen in the water many miles from land. While underwater, they keep their eyes open but close their nostrils. They can remain submerged for up to two minutes.
NUDIBRANCH
Crawling over a vivid red seafan in the Komodo National park, Indonesia, this brightly colored nudibranch, or sea slug, stands out. Its scientific name, Phyllidia pustulosa, reflects the diseaselike nodules on its back. These yellow beacons warn fish that it contains toxins and they should not eat it.
THE GREENLAND COAST
In this satellite image of part of Greenland’s eastern coast, taken during a summer thaw, the brown areas are rocky land. Penetrating it are many long fiords, some partly filled by glaciers and large, flat icebergs. Other icebergs of varying size, formed from a disintegrated ice shelf, float offshore in vast numbers.
INTRODUCTION
EARTH’S OCEANS CONTAIN about
320 million cubic miles (1.34 billion cubic kilometers) of seawater. Dissolved in this are some 53 million billion tons (48 million billion metric tons) of salts, gases, and other substances. The base substance, water itself, has many unusual properties, such as its high surface tension and heat capacity, which are of tremendous significance to everything from the oceans’ ability to support life to their stabilizing effect on the world’s climate, and their ability to transmit waves. Also of significance is the variability of ocean water—the sea is not uniform but varies spatially and sometimes seasonally in attributes such as its temperature, pressure, dissolved-oxygen content, and level and quality of light illumination. These attributes are important in numerous key respects.
OC E A N WAT E R CRASHING WAVE
This “barrel” wave is crashing onto the north shore of the island of Oahu, Hawaii. Inspiring sights such as this are only possible because of some of the unusual properties of water.
30
OCEAN WATER hydrogen atom consists of one proton and one electron
The Properties of Water
hydrogen nucleus, consisting of single proton, contains positive charge
THE MAIN CONSTITUENT OF THE OCEANS IS, of
course, water. The presence of large amounts of liquid water on Earth’s surface over much of its history has resulted from a fortunate combination of factors. Among them are water’s unusually high freezing and boiling points for a molecule of its size, and its relative chemical stability. Water also has other remarkable properties that contribute to the characteristics of oceans—from their + + ability to support life to effects on climate. Underlying these properties is water’s molecular structure. water molecule
– region of slight negative charge
hydrogen bond
The Water Molecule
hydrogen bond
+
+ –
– +
+ region of slight positive charge
–
+
shared electron
one of eight electrons in oxygen atom oxygen
atom free A molecule of water (H2O) consists of two hydrogen (H) electron atoms bound to one atom of oxygen (O). Crucial to formation of the bonds between the oxygen and hydrogen atoms are four tiny negatively charged particles called electrons, which are shared between the atoms. In addition, six other electrons move around HYDROGEN BONDS within different regions of the oxygen atom. This A hydrogen bond is an electron arrangement makes the H2O molecule attractive electrostatic chemically stable but gives it an unusual shape. It also force between regions of produces a small imbalance in the distribution of slight positive and negative charge on neighboring water electrical charge within the molecule. An important molecules. Several bonds result of this is that neighboring water molecules are are visible here. drawn to each other by forces called hydrogen bonds.
oxygen nucleus, containing protons and neutrons, has positive charge
CHARGE IMBALANCE
The distribution of negative charges (electrons) and regions of positive charge in an H2O molecule causes one side to carry a slight positive charge and the other side a slight negative charge.
water molecule at surface
WALKING ON WATER
INTRODUCTION
Certain insects, such as sea skaters and water striders (pictured below), exploit surface tension to walk, feed, and mate on the surface of the sea, lakes, or ponds.
Surface Tension
hydrogen bonds
One special property of liquid water that can be directly attributed to the attractive forces between its molecules is its high surface tension. In any aggregation of water molecules, the surface molecules tend to be drawn together and inward toward the center of the aggregation, water forming a surface “skin” that is resistant to disruption. Surface molecule tension can be thought of as the force that has to be exerted below surface or countered to break through this skin. Water’s high surface tension has various important effects. Perhaps the most crucial is that it is vital to certain processes within living organisms—for example, water transport in plants and blood transport in animals. Surface tension also allows small insects such as sea skaters to walk and feed on the ocean surface, and it even plays a part in the formation of ocean waves (see p.76).
CAUSE OF SURFACE TENSION
In a drop of water, molecules are pulled in all directions by hydrogen bonding with their neighbors. But at the surface, the only forces act inward, or sideways, toward other surface molecules.
WATER DROPLETS
The shape of these droplets results from surface tension. The forces pulling their surface molecules together are stronger than the gravitational forces flattening them.
THE PROPERTIES OF WATER LAND AND SEA
31
Heat Capacity
Water’s high heat capacity means that the Sun warms the sea more slowly than land. In this satellite-generated temperature map of south California during a heat wave, much of the land (red) is 122ºF (50ºC) or more, but the sea (left) is cool, at 50ºF (10ºC).
A second property of liquid water that can be attributed to hydrogen bonding is its unusually high heat capacity, which exceeds that of nearly all other known liquids (see table, left). When heat is added to water, most of the heat is used to break hydrogen bonds linking the molecules. Only a fraction of the energy increases the vibrations of the water molecules, which are detected as a rise in temperature. This means that areas of ocean can absorb and release huge amounts of heat energy with little change in temperature. It also means that movements of water— ocean currents—transfer enormous amounts of heat energy around the planet. This role of ocean currents is vital to Earth’s climate (see p.66).
SPECIFIC HEAT CAPACITY Specific heat capacity (SHC) is the energy (in joules) needed to raise the temperature of 1 gram of a substance by 1ºC. Listed below are the SHCs of 13 liquids, measured at room temperature unless otherwise stated. SUBSTANCE
JOULES/GRAM ºC
Liquid ammonia at –40ºF (–40ºC)
4.7
Fresh water
4.19
HEAT ON THE MOVE
Seawater at 35ºF (2ºC)
3.93
In this temperature map of part of the northwest Atlantic, the water surface ranges from about 41ºF (5oC) (blue) to 77ºF (25ºC) (red). A warm current, the Gulf Stream is visible in red.
Glycerin
2.43
Ethanol (ethyl alcohol)
2.4
Acetone
2.15
Kerosene
2.01
Olive oil
1.97
Benzene
1.8
Turpentine
1.72
Freon 12 refrigerant at -40ºF (-40ºC)
0.88
Bromine
0.47
Mercury
0.14
WATER TWISTER
The effects of surface tension can cause moving sheets, jets, and streams of water to assume or hold together in some surprising forms, as in this slightly spiral-shaped water jet.
Three States of Water The temperatures at which water changes between its three states— melting point (ice to liquid water) and boiling point (liquid water to water vapor)—are both high compared with substances having similarly sized molecules. For ice to melt and water to vaporize, high levels of energy are needed to break all the hydrogen bonds. Water is also unusual in that its solid form is slightly less dense than its liquid form, so ice floats in liquid water. The reason for this is that the molecules in ice are loosely packed, whereas those in liquid water move around in snugly packed groups. The fact that ice floats on liquid water is important because it allows the existence of large areas of polar sea ice (see pp. 198–199). These affect heat flow between ocean and atmosphere and help stabilize ocean temperatures and Earth’s climate.
SOLID, LIQUID, AND GAS ICE
LIQUID WATER
WATER VAPOR
Hexagonal crystal lattice
Small clumps of bonded molecules
Widely spaced unbonded molecules
In ice, hydrogen bonds hold the water molecules together in a rigid structure. In liquid water, the bonds hold the molecules in small, moving clumps. In water vapor, there are no hydrogen bonds.
Water is the only natural substance found in all three states at Earth’s surface. Sometimes, ice, liquid water, and condensing water vapor can be seen side by side, as here at the fjord in Spitsbergen.
INTRODUCTION
SIDE BY SIDE
32
OCEAN WATER
The Chemistry of Seawater
volcanic ash drifts down to sea
THE OCEANS CONTAIN MILLIONS OF DISSOLVED
chemical substances. Most of these are present in exceedingly small concentrations. Those present in significant concentrations include sea salt, which is not a single substance but a mixture of charged particles called ions. Other constituents include gases such as oxygen and carbon dioxide. One reason the oceans contain so many dissolved substances is that water is an excellent solvent.
The Salty Sea
salts are leached from rocks into rivers and streams and flow to ocean
The salt in the oceans exists in the form of charged particles, called ions, some positively charged and some negatively charged. The most common of these are sodium and chloride ions, the components of ordinary table salt (sodium chloride). Together they make up about 85 percent by mass of all the salt in the sea. Nearly all the rest is made up of the next four most common ions, which are sulfate, magnesium, calcium, and potassium. All these ions, together with several others present in smaller quantities, exist throughout the oceans in fixed proportions. Each is distributed extremely uniformly—this is in contrast to some other dissolved substances in seawater, which are unevenly distributed.
salt spray onto land
nutrients from soil wash into rivers and streams, and flow to ocean
BREAKDOWN OF SALT
If 2½ gallons (10 liters) of seawater are evaporated, about 123/4 oz (354 g) of salts are obtained, of the types shown below. 2½ gallons (10 liters) of seawater
other salts 1/4oz (7.5g) calcium sulfate (gypsum) 2/3oz (17.7g) magnesium salts 2oz (54.8g) sodium chloride (halite) 10oz (274g)
+ +
+ – –
+
– Na+
–
–
– –
+
WATER AS A SOLVENT
+
slow uplift of sedimentary rocks at continental margins, exposing salts, minerals, and ions at surface
–
sodium chloride crystal
+ –
+
+ Cl–
+
water molecule
+
ALEXANDER MARCET The Swiss chemist and doctor Alexander Marcet (1770–1822) carried out some of the earliest research in marine chemistry. He is best known for his discovery, in 1819, that all the main chemical ions in seawater (such as sodium, chloride, and magnesium ions) are present in exactly the same proportions throughout the world’s oceans. The unchanging ratio between the ions holds true regardless of any variations in the salinity of water and is known today as the principle of constant proportions.
+ –
+ + –
–
PEOPLE
uptake of nutrients by phytoplankton
–
–
chloride ion (negative charge)
INTRODUCTION
+
–
+
The charge imbalance on its molecules makes water a good solvent. When dissolving and holding sodium chloride in solution, the positive ends of the molecules face the chloride ions and the negative ends face the sodium ions.
sodium ion (positive charge)
–
Sources and Sinks
nutrient upwelling exchange of gases between phytoplankton and seawater
sinking and
decomposition The ions that make up the salt in the oceans have arrived of dead there through various processes. Some were dissolved out of organisms rocks on land by the action of rainwater and carried to the sea in rivers. Others entered the sea in the emanations of hydrothermal vents (see p.188), in dust blown off the land, or came from volcanic ash. There are also “sinks” for every type of ion—processes that remove them from seawater. These range from salt spray onto land to the precipitation of various ions onto the seafloor as mineral deposits. Each type of ion has a characteristic residence time. This is the time that an ion remains in seawater before it is removed. The common ions in seawater have long residence times, ranging from a few hundred years to hundreds of millions of years.
RIVER DISCHARGE
River discharge is a mechanism by which ions of sea salt and nutrients enter the oceans. Here, the Noosa River empties into the sea on the coast of Queensland, Australia.
THE CHEMISTRY OF SEAWATER SOURCES, SINKS, AND EXCHANGES
spread of volcanic ash and gases into rain clouds
Shown here are various sources, sinks, and exchange processes for the ions, salts, and minerals (yellow arrows), gases (pink arrows), and plant nutrients (turquoise arrows) in seawater.
KEY
33
Gases in Seawater
gases ions, salts, and minerals plant nutrients
The main gases dissolved in seawater are nitrogen (N), oxygen (O2), and carbon dioxide (CO2). The levels of O2 and CO2 vary in response to the activities of photosynthesizing organisms (phytoplankton) and animals. The level of O2 is generally highest near the surface, where the gas is absorbed from the air and also produced by photosynthesizers. Its concentration drops to a minimum in a zone between about 660 ft (200 m) and 3,300 ft (1,000 m), where oxygen is consumed by bacterial oxidation of dead organic matter and by animals feeding on this matter. Deeper down, the O2 level increases again. CO2 levels are highest at depth and lowest at the surface, where the gas is taken up by photosynthesizers faster than it is produced by respiration. CARBON SINK
washing of ions from volcanic dust and gases into sea, dissolved in rain
Many marine animals, such as nautiluses (below), use carbonate (a compound of carbon and oxygen) in seawater to make their shells. After they die, the shells may form sediments and eventually rocks.
dust blown off land
exchange of gases between animals and seawater
exchange of gases between ocean and atmosphere
OXYGEN PRODUCER AND CONSUMER
Oxygen levels in the upper ocean depend on the balance between its production by photo-synthesizing organisms, such as kelp, and its consumption by animals, such as fish.
Nutrients
release of minerals from hydrothermal vents dissolving of minerals from sea floor precipitation of minerals onto sea floor
PLANKTON BLOOM
This satellite image of the Skagerrak (a strait linking the North and Baltic seas) shows a bloom of phytoplankton, visible as a turquoise discoloration in the water.
SILICEOUS DIATOMS
These tiny forms of planktonic organisms have cell walls made of silicate. They can only grow if there are sufficient amounts of silica present in the water.
INTRODUCTION
carbonates incorporated into seafloor sediments from animal shells
Numerous substances present in small amounts in seawater are essential for marine organisms to grow. At the base of the oceanic food chain are phytoplankton—microscopic floating life-forms that obtain energy by photosynthesis. Phytoplankton need substances such as nitrates, iron, and phosphates in order to grow and multiply. If the supply of these nutrients dries up, their growth stops; conversely, blooms (rapid growth phases) occur if it increases. Although the sea receives some input of nutrients from sources such as rivers, the main supply comes from a continuous cycle within the ocean. As organisms die, they sink to the ocean floor, where their tissues decompose and release nutrients. Upwelling of seawater from the ocean floor (see p.60) recharges the surface waters with vital substances, where they are taken up by the phytoplankton, refueling the chain.
34
OCEAN WATER
Temperature and Salinity OCEAN WATER IS NOT UNIFORM BUT VARIES
in several physical attributes, including temperature, salinity, pressure, and density. These vary vertically (dividing the oceans into layers), horizontally (between tropical and temperate regions, for example), and seasonally. The basic variables, temperature and salinity, in turn produce variations in density that help drive deep-water ocean circulation.
Temperature Temperature varies considerably over the upper areas of the oceans. In the tropics and subtropics, solar heating keeps the ocean surface warm throughout the year. Below the surface, the temperature declines steeply to about 8–10˚C (46-50˚F) at a depth of 1,000m (3,300ft). This region of steep decline is called a thermocline. Deeper still, temperature decreases more gradually to a uniform, near-freezing value of about 2˚C (36˚F) on the sea floor – this temperature subsists throughout the deep oceans. In mid-latitudes there is a much more marked seasonal variation in surface temperature. In high latitudes and polar oceans, the water is constantly cold, sometimes below 0˚C (32˚F).
LA NIÑA TEMPERATURE ANOMALY IN PACIFIC
The Pacific experiences long-term fluctuations in the temperature patterns of its surface waters, which are linked to climatic disturbances known as El Niño and La Niña. This visualization, based on satellite data, shows a strong La Niña-type surface temperature pattern that developed in late 2010, with a blue band indicating lower than normal temperatures in the eastern Pacific.
cool surface waters caused by cold current moving up coast
OCEAN SURFACE TEMPERATURE
This map shows average surface temperatures in March. Proximity to the equator is the main factor determining surface temperature, but ocean currents also play a role.
warm tropical water, with temperatures constantly above 25˚C (77˚F) region of variable surface temperature, fluctuating seasonally from 7 to 20˚C (45–68˚F)
constantly cold water with temperatures in the range 0–3˚C (32–37˚F)
North America
constantly cold water off Greenland
constantly warm pool of water in Caribbean Sea
INTRODUCTION
warm surface waters caused by warm current moving down southeast coast
thermocline, where temperature declines rapidly with depth
South America
TEMPERATURE AND DEPTH
cold bottom water at a uniform temperature of 2˚C (36˚F)
Shown in early summer, the vast bulk of ocean water in this part of the north Atlantic is uniformly cold (below 5˚C/41˚F). Only a thin surface layer from the tropics into mid- latitudes is warmed above this base level.
KEY
90°F
32°C 30°C
70°F
20°C
50°F
10°C
30°F
0°C
35
Salinity
Pressure
Salinity is an expression of the amount of salt in a fixed mass of seawater. It is determined by measuring a seawater sample’s electrical conductivity and averages about 35 grams of salt per kilogram of seawater. Salinity varies considerably over the surface of oceans – its value at any particular spot depends on what processes or factors are operating at that location that either add or remove water. Factors that add water, causing low salinity, include high rainfall, river input, or melting of sea-ice. Processes that remove water, causing high salinity, include high evaporative losses and sea-ice formation. At depth, salinity is near constant throughout the oceans. Between the surface and deep water is a region called a halocline, where salinity gradually increases or decreases with depth. Salinity affects the freezing point of seawater – the higher the salinity, the lower the freezing point.
Scientists measure pressure in units called bars. At sea level, the weight of the atmosphere exerts a pressure of about one bar. Underwater, pressure increases at the rate of one bar for every 10m (33ft) increase in depth, due to the weight of the overlying water. This means that at 70m (230ft), for example, the total pressure is eight bars or eight times the surface pressure. This pressure increase poses a challenge to human exploration of the oceans. To inflate their lungs underwater, divers have to breathe pressurized air or other gas mixtures, but doing DECOMPRESSION STOP so can cause additional To avoid a condition called “bends” that can arise from problems (arising from the dissolution of excess decompressing too quickly, on their way to the surface gas in body tissues). scuba divers make one or These problems limit more timed stops to release excess gas. the depths attainable.
NATURAL ADAPTATION
Elephant seals can dive to depths of up to 1,550m (5,100ft). They have evolved various adaptations for coping with the high pressure, including collapsible ribcages.
KEY
EASY FLOATING
GLOBAL SALINITY
In some enclosed seas where evaporative losses are high and there is little rainfall or river inflow, the sea- water can become so saline and dense that floating becomes easy. This is the case here in the Dead Sea.
Surface salinity is highest in the subtropics, where evaporative losses of water are high, or in enclosed or semi-enclosed basins (such as the Mediterranean). It is lowest in colder regions or where there are large inflows of river water.
37 36 35 34 33 32 31 30 29 under 29 parts per thousand (‰)
Density
DECOMPRESSION After working underwater for hours at a time, professional divers routinely undergo controlled decompression in a purpose-built pressure chamber. These facilities are also used to treat pressure-related diving illnesses and for research into diving physiology. PRESSURE CHAMBER
The person being decompressed may have to breathe a special gas mixture while the ambient pressure is slowly reduced.
warm surface flow
Atlantic Central Water: warm, low-density surface waters in the tropics and subtropics
Atlantic Intermediate Water: cool layer of intermediate density, forms and sinks in north Atlantic, then moves south
DENSITY LAYERS IN ATLANTIC
The oceans each contain distinct, named water masses that increase in density from the surface downwards. The denser, cooler masses sink and move slowly towards the Equator. The cold, high-density deep and bottom waters comprise 80 per cent of the total volume of the ocean.
Antarctic Bottom Water: coldest and densest layer, forms close to Antarctica, sinks then moves north
mid-ocean ridge
North Atlantic Deep Water: cold, dense water, forms and sinks in north Atlantic, then moves south
INTRODUCTION
The density of any small portion of seawater depends primarily on its temperature and salinity. Any decrease in temperature or increase in salinity makes seawater denser – an exception being a temperature drop below 4˚C (39˚F), which actually makes it a little less dense. In any part of the ocean, the density of the water increases with depth, because dense water always sinks if there is less dense water below it. Processes that change the density of seawater cause it to either rise or sink, and drive large-scale circulation in the oceans between the surface and deep water (see p.60). Most important is water carried towards Antarctica and the Arctic Ocean Antarctic Intermediate fringes. This becomes denser as it Water: cool layer intermediate cools and through an increase in of density, sinks and its salinity as a result of sea-ice moves north formation. In these regions large quantities of cold, dense, salty water continually form and sink towards the ocean floor.
DISCOVERY
36
OCEAN WATER
Light and Sound LIGHT AND SOUND BEHAVE VERY DIFFERENTLY
DEPTH
Violet
Blue
Green
570nm 400nm
510nm
60m
Yellow
(100ft)
Light in the Ocean 590nm
650nm
30m
Orange
Red
in water than in air. Most light wavelengths are quickly absorbed by water, a fact that both explains why a calm sea appears blue and why ocean life is concentrated near its surface – almost the entire marine food chain relies on light energy driving plant growth. Sound, in contrast, travels better in water, a fact exploited by animals such as dolphins.
(200ft)
475nm
90m (300ft)
LIGHT PENETRATION
The red and orange components of sunlight are absorbed in the top 15m (50ft) of the ocean. Most other colours are absorbed in the next 40m (130ft). Wavelength is measured in nanometres (nm).
White light, such as sunlight, contains a mixture of light wavelengths, ranging from long (red) to short (violet). Ocean water strongly absorbs red, orange, and yellow light, so only some blue and a little green and violet light reach beyond a depth of about 40m (130ft). At 90m (300ft), most of even the blue light (the most penetrating) has been absorbed, while below 200m (650ft), the only light comes from bioluminescent organisms, which produce their own light (see p.224). Because they rely on light to photosynthesize, phytoplankton are restricted to the upper layers of the ocean, and this in turn affects the distribution of other marine organisms. Intriguingly, many bright red animals live at depths that are devoid of red light: their colour provides effective camouflage, since they appear black.
COLOUR RESTORATION
At a depth of 20m (65ft), most animals and plants look blue-green under ambient light conditions (top). Lighting up the scene with a photographic flash or torch reveals the true colours of the marine life (bottom).
INTRODUCTION
FISH VISION
FIREFLY SQUID
This squid produces a pattern of glowing spots (photophores). When viewed by a predator swimming below, the spots help camouflage its outline against the moonlit waters above.
Fish have excellent vision, which helps them f ind food and avoid predators. Many can see in colour. The lens of a fish’s eye is almost spherical and made of a material with a high refractive index. It can be moved backwards and forwards to focus light on the retina. FISH EYE
The lens of a fish’s eye bulges through the iris (the dark central part) almost touching the cornea (outer part). This helps to gather the maximum amount of light and gives a wide field of view.
LIGHT AND SOUND
Seen from underwater, only a part of the surface of the sea appears lit up, while the rest looks dark. This is an effect of the way light waves are bent (refracted) when they enter the sea from the air.
Sea Colours Seawater has no intrinsic colour – a glass of seawater is transparent. But on a clear, sunny day, the sea usually looks blue or turquoise. In part, this is due to the sea surface reflecting the sky, but the main reason is that most of the light coming off the surface has already penetrated it and been reflected back by particles in the water or by the sea bed. During its journey through the water, most of the light is absorbed, except for some blue and green light, which are the colours seen. Other factors can modify the sea’s colour. In windy weather, the surface becomes flecked with white, caused by trapped bubbles of air, which reflect most of the light that hits them. Rain interferes with seawater’s light-transmitting properties, so rainy, overcast weather generally produces dark, grey-green seas. Occasionally, living organisms, such as “blooms” of plankton can turn patches of the sea vivid colours.
VIVID GREEN FROM ALGAL BLOOM
TROPICAL TURQUOISE
OCEAN SHADES
A green sea (top) is sometimes caused by the presence of algae. Turquoise is the usual shade in clear tropical waters, while grey water flecked with white foam is typical of windy, overcast days.
GREY FOAMY TEMPERATE SEA
Underwater Sounds
PEOPLE
WALTER MUNK The Austrian-American scientist Walter Munk (b.1917) pioneered the use of sound waves in oceanography. A professor at the Scripps Institute of Oceanography in San Diego, California, Munk demonstrated that by studying the patterns and speed of sound propagation underwater, information can be obtained about the large-scale structure of ocean basins.
The oceans are noisier than might be imagined. Sources of sound include ships, submarines, earthquakes, underwater landslides, and the sounds of icebergs breaking off glaciers and ice shelves. In addition, by transmitting sound waves or bouncing them off underwater objects (echolocation) whales and dolphins use sound for navigation, hunting, and communication. Sound waves travel faster and further underwater than they do in air. Their speed underwater is about 1,500m (5,000ft) per second and is increased by a rise in the pressure (depth) of the water and decreased by a drop in temperature. Combining these two effects, in most ocean regions, there is a layer of minimum sound velocity at a depth of about 1,000m (3,300ft). This layer is called the SOFAR (Sound Fixing and Ranging) channel. The properties of the SOFAR channel are exploited by people using underwater listening devices and, it has been theorized, by animals such as whales and dolphins. HUMPBACK WHALE SONG
The peaks and troughs in this spectrogram show the changes in frequency of a few seconds of repeated sound made by a Humpback Whale. THE SOFAR CHANNEL
sound travels slower within channel
1,000m
SOFAR channel
(3,300ft) 2,000m (6,600ft) 3,000m (9,800ft) 1,500m/s 1,525m/s 1,550m/s (4,900ft/s) (5,000ft/s) (5,085ft/s)
SPEED OF SOUND UNDERWATER
Low-frequency sounds generated in the SOFAR channel are “trapped” in it by inward refraction from the edges of the channel. As a result, sounds can travel very long distances in this ocean layer.
INTRODUCTION
Sea level
DEPTH
LOOKING UP
37
OCEANS ARE ALMOST as old as
Earth itself. Sediments were probably accumulating underwater about 4 billion years ago, around the time that the oldest known rocks on Earth were forming. And yet the ocean floor is very young. Discovery of the processes that create and rapidly recycle the ocean floor led to our modern understanding of plate tectonics. These processes give Earth a surface quite unlike those of our planetary neighbors, with deep ocean basins and high-standing continents. The positions of the oceans and continents are not fixed, but driven by heat flow deep within the planet. An understanding of ocean geology opens a window on Earth’s interior, as well as providing insights into the global climate and the evolution of life on Earth.
OC EA N G E O L O G Y OCEANIC LAVA
Steam mixes with surf as lava from Kilauea Crater reaches the Pacific Ocean on the south shore of Hawaii. Basaltic lava such as this makes up the oceanic part of Earth’s crust.
40
OCEAN GEOLOGY
The Formation of the Earth THE EARTH STARTED TO FORM MORE THAN
4,500 million years ago in a disk of gas, dust, and ice around the early Sun. This protoplanetary disk, as it is known, was held in orbit by the gravitational field of the young star. Gravitational attraction between dust particles in the disk produced small rocks, and collisions concentrated the rocks into several rings. The most densely populated rings went on to form the planets of the Solar System.
EARLY SOLAR SYSTEM
Birth of the Earth
The early Solar System contained a disk of dust, ice, and gas, from which the rocky inner planets and gaseous outer planets formed.
small pieces of rock and ice pulled together by gravitational attraction
planetesimals start to form in protoplanetary disk around Sun
Initially, the rocks within each ring drifted together, due to their mutual gravitational attraction, in a process known as cold accretion. The largest bodies in each ring attracted the most material and grew to form objects larger than 0.7 mile (1 km) across, called planetesimals. Planetesimals are loose collections of rock and ice, with a uniform structure. As the mass of a planetesimal grows larger, it exerts a stronger gravitational pull, becoming more tightly held together and attracting nearby rocks with greater force. Collisions between planetesimals broke them apart or grouped them together. In the inner Solar System, the planetesimals in each orbiting ring came together to form much larger objects, called protoplanets, and these later collided to form the rocky planets. The Earth was born in this way about 4.560 million years ago.
1 COLD ACCRETION
Under gravity, pieces of rock and ice coalesced. Material sharing the same orbit around the Sun clumped together to form planetesimals.
3 HEAVY BOMBARDMENT
Each growing protoplanet attracted more planetesimals, which impacted through more energetic, high-speed collisions. Finally the protoplanets themselves underwent a series of collisions to form the rocky planets, including Earth.
2 PROTOPLANET
By attracting more clumps of rock and ice, and through many collisions, planetesimals grew into protoplanets. As the size of these increased, gravity smoothed out their surfaces.
rocks accelerate toward primordial Earth
impacts generate surface heat and local melting
Internal Heat The early Earth was hot but mostly solid, although with a partially molten surface, and had a fairly uniform internal composition.Today, it has layers of different compositions, including a dense, partially liquid core of iron and nickel.The transition may have its roots in several different energy sources. Localized surface melting would have occurred when the kinetic energy of incoming rocks was converted to heat during impacts. More significant heat sources would have been the decay of radioactive elements in the interior rocks and the heat released by the Earth’s contraction under the force of its own gravity—a process that led to an event called the iron catastrophe (see below). Impact with a sufficiently large body might have released enough heat to melt the Earth’s interior, and this may have happened more than once.
impact of Mars-sized body leads to total melting
MOON FORMATION
It is thought that early in Earth’s history, it was struck by a large protoplanet, creating the Moon, tilting the Earth’s axis of rotation, and leaving it with a slightly eccentric orbit.
INTRODUCTION
THE IRON CATASTROPHE
material ejected during collision later cooled and coalesced to form the Moon
1
As the Earth grew larger, the strength of its gravitational field increased, which in turn attracted more material.
2
Eventually, the gravitational field was strong enough to cause the Earth to contract, converting gravitational potential energy into heat.
3
Enough heat was released to melt the iron contained in the Earth’s rocks, allowing it to flow down to the center of the Earth.
4
The sinking of large amounts of iron released further heat, enough to melt the entire interior of the planet in the event called the iron catastrophe.
THE FORMATION OF THE EARTH
41
A LAYERED EARTH
Convection and Differentiation After the interior of the Earth melted, its heaviest constituents were able to sink to the center and the lighter ones to rise toward the surface. One-third of the planet’s mass pooled at the center and formed a dense core consisting mainly of iron, the heaviest of the common elements making up the Earth. The core became the hottest part of the planet, up to 11,700˚F (6,500˚C), and a source of heat for the molten rocks above. Most materials expand as they are heated, becoming less dense and more buoyant. This is the basis of convection, which provided a mechanism for carrying heat and material from the interior of the Earth toward the surface.Vigorous convection cells carried hot, buoyant material upward, where it lost heat by conduction near the surface, before sinking again. Lighter materials such as aluminum were left behind at the surface, forming a thin crust. In this way, the Earth became differentiated into layers of different chemical composition: a metallic core, a rocky mantle, and a buoyant crust. This occurred as early as 4,500 million years ago.
The early Earth had a uniform composition but melting allowed chemical “zoning” to develop. convection carries internal heat to surface
lighter materials rise up through semi-fluid mantle
carbon dioxide
water vapor
heavy materials sink to form dense core nitrogen
ATMOSPHERE AND OCEAN
The lightest materials of all, gases and water, were expelled from the interior to form the outer atmospheric and ocean layers at an early stage in the Earth’s history.
The Earth Today The Earth’s interior is now split into three chemically distinct layers, which can be further differentiated by changes in their physical properties due to temperature and pressure variations with depth. The core consists of an iron-nickel alloy, with some impurities, at a temperature of 7,200–11,700˚F (4,000–6,500˚C). Iron in the inner part of the core has solidified under the immense pressure, but the outer part is still a free-flowing liquid. The mantle of silicate rock surrounding the core has also solidified, but a form of convection called “solid-state creep” still takes place, with material in the lower mantle moving a few inches per year. The upper mantle, within about 255 miles (410 km) of the surface, is a more easily deformed “plastic” region. Above it floats a thin crust enriched in lighter elements, with average thickness ranging from 5 miles (8 km) beneath the oceans to 28 miles (45 km) beneath the continents. INSIDE THE EARTH
Earth has a layered internal structure, the main layers being the core, mantle, and crust. The density and temperature of the layers increases with depth. Heat from the core flows through the mantle, eventually reaching the cooler crust, where it escapes.
atmosphere
the transition zone is slightly denser than the upper mantle and forms a distinct layer between upper and lower mantle reservoir of magma (hot, melted rock) under Yellowstone Park, on North American Plate
hotspot under Hawaii, probably caused by a plume of hot material rising from deep in the mantle
liquid outer core solid inner core
lower mantle
upper mantle
consisting of the uppermost layer of the upper mantle together with overlying crust, the rigid lithosphere makes up tectonic plates
continental crust
the Chile Rise is a ridge marking the divergence of two tectonic plates, associated with upwelling of hot material from upper mantle
INTRODUCTION
oceanic crust
42
OCEAN GEOLOGY
The Origin of Oceans and Continents EARTH’S OCEANS FORMED MORE THAN
4 billion years ago, mainly from water vapor that condensed from its primitive atmosphere but also from water brought from space by comets. Initially, after acquiring a layered internal structure, the Earth had a uniform crust that was enriched in lighter elements and floated on an upper mantle made of denser materials. Later, the crust became differentiated into two types as continents began to form, made from rocks that were chemically distinct from those underlying the oceans.
zircon crystals, among the earliest continental crust materials ZIRCON primitive continental crust thickens above sinking mantle flow, without mantle interference
Continental Crust The continents include a wide range of rock THE OLDEST ROCKS types, including granitic igneous rocks, sedimentary These sedimentary rocks on Baffin Island rocks, and the metamorphic rocks formed by the lie on the Canadian alteration of both. They contain a lot of quartz, a Shield. The stable mineral absent in oceanic crust. The first continental continental shields rocks were the result of repeated melting, cooling, contain the world’s most ancient rocks, and remixing of oceanic crust, driven by volcanic activity above mantle convection cells, which were which are around 4 billion years old. much more numerous and vigorous than today’s. Each cycle left more of the heavier components in the upper mantle and concentrated more of the lighter components in the crust. The first microcontinents grew as lighter fragments of crust collided and fused. Thickening of the crust led to melting at its base and underplating with granitic igneous rocks. Weathering accelerated the process of continental rock formation, retaining the most resistant components, such as quartz, while washing solubles into the ocean. basaltic lava
rift
basalt sheets (dikes) sediment ocean surface
ocean crust
gabbro peridotite
lithosphere
Moho asthenosphere
magma rises to surface
top layer of upper mantle
INTRODUCTION
OCEAN-FLOOR STRUCTURE
Three layers of basalt in the crust (basaltic lava, dikes, and gabbro) are separated from the mantle by the Mohorovicˇic´ discontinuity (the Moho). The top layer of the upper mantle is fused to the base of the crust to form the rigid lithosphere, which makes up tectonic plates.The asthenosphere is the soft zone over which the plates of the lithosphere glide. MANTLE ROCKS
Peridotite is the dominant rock type found in the mantle, consisting of silicates of magnesium, iron, and other metals. Sometimes it is brought to the surface when parts of the ocean floor are uplifted, as here in Newfoundland, Canada, or as fragments from volcanic activity.
sedimentary rocks
primitive oceanic crust
volcanic activity adds igneous rocks to surface above rising flows
Oceanic Crust The oceanic crust has a higher density than the continental crust, making it less buoyant. Both types of crust can be thought of as floating on the “plastic” upper mantle, and the oceanic crust lies lower due to its lower buoyancy. It is relatively thin, with a depth of never more than 7 miles (11 km), compared with a thickness of 15–43 miles (25–70 km) for most continental crust. It consists mainly of basalt, an igneous rock that is low in silica compared with continental rocks, and richer in calcium than the mantle. Basalt lava is created when hot material in the upper mantle is decompressed, allowing it to melt and form liquid magma. The decompression occurs beneath rifts in the crust, such as those found at the mid-ocean ridges, and it is through these rifts that lava is extruded onto the surface to create new ocean crust.
THE ORIGIN OF OCEANS AND CONTINENTS DEVELOPMENT OF CONTINENTAL CRUST
Modification of the crust above rising mantle flows was delayed by the continuous intrusion of mantle basalt, resulting in the greenstone belts found today at the heart of each continental shield. greenstone belts above rising mantle flow basalt continuously intrudes from mantle
crust pulled apart by convective motion in mantle
43
BANDED IRON
Water and Atmosphere
Known as a banded-iron formation, this layered rock contains iron oxides that formed as the oxygen content of early oceans increased.
During the process of differentiation, volatile materials were expelled from Earth’s interior by volcanic activity. The lightest gases, such as hydrogen and helium, would quickly have been lost to space, leaving a stable atmosphere of nitrogen, carbon dioxide, and water vapor. Some of the water vapor would have condensed to form liquid water, and it seems there was a significant ocean earlier than 4 billion years ago. Some meteorites contain 15-20 percent ocean water from water and the early Earth is thought volcanic eruptions and comet to have had the same composition, impacts providing an ample source for the early ocean. More water arrived with impacting comets. It was in the ocean that free oxygen first appeared, with the arrival traces of early of photo- synthesizing life around 3.5 billion years ago. meteorite and comet
rivers erode and transport sediment
bombardment gradually erased
THE EARLY EARTH mantle vigorous convection cells in upper mantle
Earth had deep oceans from an early stage, with volcanoes and an increasing area of continental crust standing above the surface. The ocean became salty as weathering of surface rocks added minerals to the water.
rifts occur when fragments of crust move apart
volcanic eruptions add gases and water vapor to atmosphere
liquid outer core
solid inner core
This radar image shows volcanoes formed from andesite lava, whose composition is intermediate between oceanic and continental rocks.
INTRODUCTION
ANDEAN VOLCANOES
44
OCEAN GEOLOGY
The Evolution of the Oceans spreading ridge
EVER SINCE THE ATLANTIC COASTS OF SOUTH AMERICA
and Africa were accurately charted, it has been apparent that they match like the pieces of a jigsaw puzzle. We now know that the continents move, that they were once joined together, and that today’s oceans arose when the landmasses split apart. The evolving oceans have modified the global climate, and sea level has fluctuated in response to climate change and geological factors.
continent carried on plates
1. CAMBRIAN (500 MYA)
Plate Tectonics
The remains of the first supercontinent, Rodinia,
were scattered, with the largest piece, Gondwana, The numerous convection cells (see p.41) that gave rise to the lying in the south. The Iapetus Ocean separated first fragments of continental crust gradually gave way to fewer, Laurentia (North America) from Baltica (northern convection larger-scale convection cells as the mantle cooled. The continental Europe). The Panthalassic Ocean occupied cell drives fragments became consolidated into larger areas, and rifts most of the Northern plate motion Hemisphere. formed at the thinnest parts of the ocean crust, splitting it into large plates. When the density of the oceanic and continental plates became PANTHALASSIC OCEAN sufficiently different, the oceanic crust PLATE MOVEMENT sank where it met the more buoyant Crustal plates move continental crust, creating subduction around under the zones. Since then, the evolution of influence of convection LAURENTIA cells, which probably the oceans and continents has been reach deep down dominated by plate tectonics (see SIBERIA to the boundary pp.48–49). As the plates move, they between the outer IAPETUS carry the continents with them, with core and the mantle. OCEAN oceans opening and closing in between. BALTICA GONDWANA subduction zone
“ancestral” North Atlantic lies between North America and Europe
SIBERIA
PANTHALASSIC OCEAN
scattered remnants of Rodinia
2. DEVONIAN (400 MYA) AUSTRALIA
EURAMERICA
RHEIC OCEAN
GONDWANA
The Rheic Ocean opened when a string of islands, which were to become western and southern Europe, broke away from Gondwana and moved toward Euramerica, closing the Iapetus Ocean in the process.
shallow continental -shelf seas Ural Mountains
southern Europe joins Euramerica (Laurentia and Baltica) as Iapetus Ocean closes
first plants on land form vegetated areas
SIBERIA
PANTHALASSIC OCEAN
PALEOTETHYS SEA
INTRODUCTION
Through the Ages As Earth’s plates have moved around, largely driven by the spreading ridges and subduction zones of the rapidly recycling oceanic crust (see p.48), continents have come together and moved apart—periodically grouping together to form “supercontinents.” The German scientist Alfred Wegener proposed that 250 million years ago (mya) there was a supercontinent called Pangaea, centered on the equator and surrounded by one great ocean. It seems there was another grouping about 1 billion years ago called Rodinia, and perhaps an earlier grouping before that. Each time the continental landmasses have come together, they have eventually been broken apart as deep rifts opened up in their interiors, as is happening today in the Red Sea and the East African Rift. Computer models of the crustal fragments and the locations of spreading and subduction have allowed fairly reliable reconstructions of the geography of earlier times back to 500 million years ago.
PANGEA SOUTH AMERICA
extensive deserts
AUSTRALIA
AFRICA
GONDWANA southern ice cap covers most of South America, Africa, and Australia
3. CARBONIFEROUS (300 MYA) As the supercontinent Pangaea came together, continental masses stretched from pole to pole, almost encircling the Paleo-Tethys Sea to the east. Today’s coal seams were laid down in swampy forests along the shores of equatorial shelf seas. An extensive ice cap built up as Gondwana moved over the South Pole.
KEY
subduction zone spreading ridge outline of modern landmass
THE EVOLUTION OF THE OCEANS
45
Epicontinental Seas
PEOPLE
ALFRED WEGENER
At most times in the past, sea levels have been higher than they are today. This has given rise to shallow, tideless bodies of water called epicontinental seas covering extensive parts of the continental interiors. These were quite unlike the deep ocean basins and continental-shelf seas familiar to us today. The area of dry land was sometimes reduced to half its current extent by these seas, which were often very salty, low in oxygen, and devoid of life. They could isolate parts of continents, causing populations of living things to evolve separately. Epicontinental seas also affected the climate: their high salinity produced downwelling (see p.60) of dense water into adjacent equatorial oceans, in contrast to the polar downwelling that dominates the deep-ocean circulation today.
Alfred Wegener (1880–1930) was a German scientist with interests in astronomy, meteorology, and geology. In 1915 he presented the theory of continental drift to explain the presence of identical rocks on opposite sides of the Atlantic Ocean and tropical plant fossils in the Arctic Circle. His ideas were not accepted until seafloor spreading was discovered, providing a mechanism to explain his theory.
SHALLOW WATER
4. JURASSIC (150 MYA)
Conditions on the shore of North America’s Western Interior Seaway 100 million years ago may have been similar to the shallow lagoons of the Bahama Islands today (right).
central Atlantic starts to open
The Paleo-Tethys Sea closed as future parts of central Asia broke away from Gondwana and moved north, with the Tethys Ocean opening up behind them. The central Atlantic was opening, splitting Pangaea into northern and southern components.
LAURASIA NORTH AMERICA
PAC I F I C OCEAN
ASIA
EUROPE
TETHYS OCEAN
AFRICA SOUTH AMERICA
GONDWANA AUSTRALIA
opening of north Atlantic splits apart Europe and North America
ANTARCTICA
rifting signals creation of floor of modern Pacific Ocean
Western Interior Seaway
high sea levels
ARCTIC OCEAN
polar ice cap lost NORTH AMERICA
ASIA EUROPE
5. CRETACEOUS (100 MYA) The break-up of Gondwana started with India, Africa, and Antarctica rifting apart. This also started the closure of the Tethys Ocean. The opening of the south Atlantic soon followed, Europe separated from North America, and the Arctic Ocean opened over the North Pole.
PAC I F I C OCEAN SOUTH AMERICA
INDIA AUSTRALIA
Turgai Seaway
ANTARCTICA
Gondwana breaks up
Isthmus of Panama yet to close
remnants of Tethys Ocean EUROPE
ASIA
6. EOCENE (50 MYA) AFRICA INDIA
SOUTH AMERICA
INDIAN OCEAN AUSTRALIA
Antarctic ice cap begins to form
ANTARCTICA
India continued its rapid movement north, which would end with the uplift of the Himalayas when it hit Asia. Africa’s convergence with Europe closed the western Tethys Ocean. Australia and South America both separated from Antarctica, allowing the establishment of the Circumpolar Current that isolated Antarctica from equatorial heat flow. Australia moves north
INTRODUCTION
ATLANTIC OCEAN PAC I F I C OCEAN
TETHYS OCEAN
AFRICA
46
OCEAN GEOLOGY
Currents, Continents, and Climate Along with the atmosphere, the oceans are the means by which heat is redistributed around the Earth. Most energy arriving from the Sun is absorbed as heat near the Equator. It is then redistributed to colder regions. About 40 per cent of the heat reaching the poles from the Equator comes via ocean currents. The pattern of circulation in the oceans therefore has a large influence on the Earth’s climate (see pp.66–67). As continents, oceans, and currents have shifted through geological time, major climate changes have occurred. Conversely, warmer and colder periods affect sea level and the extent of seas.There is even speculation that the ocean froze to a depth of 2,000m (6,500ft) in places during a series of “snowball” events 775–635 million years ago, and possibly earlier, each event lasting up to 15 million years.
During snowball events, global glaciation would have left only the peaks of the highest mountains free of ice, as is the case today in Antarctica.
Greenhouse to Icehouse
MESOZOIC CURRENTS
100 million years ago, ocean currents flowed through a continuous seaway from the Tethys Ocean in the east, through what is now the Mediterranean, the Central Atlantic between North and South America, and into the Pacific in the west.
During the Mesozoic Era (252–65 million years ago) the climate was warmer than it is today, with a more even temperature distribution and no polar ice caps. Ocean currents freely flowed around the Equator, absorbing energy as they went, and carried heat to higher latitudes. The transition from this “greenhouse” climate to today’s cooler “icehouse” is due to shifts in ocean currents following the breakup of Gondwana. When the other continents moved north, the Antarctic was surrounded by the Circumpolar Current, blocking heat flow from the Equator. Equatorial flow between the oceans finally stopped when the Isthmus of Panama closed 5–3 million years ago. Antarctica now lies over the South Pole, allowing snow to accumulate into a thick ice cap, which reflects energy rather than absorbing it.
TODAY’S CIRCULATION
Today, equatorial ocean currents are blocked by landmasses, and the South Circumpolar Current is the strongest current, blocking heat flow to the South Pole. The polar regions are colder.
English Channel land bridge
Beringia land bridge
Greenland Ice Sheet Cordilleran Ice Sheet
Laurentide Ice Sheet
Patagonian Ice Sheet
Gulf of Persia dry Siberian Ice Sheet
Scandinavian Ice Sheet
sea ice
Antarctic Ice Sheet
INTRODUCTION
SNOWBALL EARTH
Sunda land bridge
Sahul land bridge
Yellow Sea dry
LAST GLACIAL (21,500 YEARS AGO)
Earth’s climate swings between ice ages and warmer periods over cycles lasting 100,000 years or more. Within ice ages, there are colder periods called glacials and warmer periods called interglacials. During glacials (the last of which peaked 21,500 years ago), the world’s ice-sheets expand, lowering global sea levels and revealing land bridges.
18/64 16/61
PRESENT LEVEL AT 0
20/68
tac eou s Pal eog e ne Ne og ene SEA LEVEL M/FT
22/72
14/57 300/980
12/54 TEMPERATURE ˚C/˚F
change of 100–200m (330–655ft) over a few tens of thousands of years. The rate of sea-floor spreading also affects global sea levels and has outweighed climatic factors at some times. Faster-spreading ridges reduce the volume of the ocean basins as the younger, hotter crust rises higher, causing sea levels to rise (see p.88). Local changes also occur as a result of crustal movement.
MEDITERRANEAN BASIN HISTORY
100/330 0/0
0
50
100
150
200
250
300
350
400
450
-100/-330 500
542
Sea level has constantly changed through history, being up to 400m (1,300ft) higher in the past. One of the factors controlling sea level is the global climate. Thermal expansion of ocean water increases global sea level by about 7.5cm (3in) for every 1˚C (1.8˚F) increase in temperature. The transfer of water between ice caps and the oceans during glacial cycles accounts for a global
200/660
10/50
47
Sea-level Change
Cre
Ca mb ria n O rd ovi c i an Sil uri an De von ian Ca rbo nif ero us Per mi an Tria ssi c Jur ass ic
THE EVOLUTION OF THE OCEANS
MILLIONS OF YEARS AGO
TEMPERATURE AND SEA LEVEL
Over the last 100 million years, climate has controlled sea levels, and these (in blue on graph) have dropped as the climate has cooled (temperature in yellow). At other times, low sea levels were due to reduced rates of sea-floor spreading.
The Mediterranean was isolated from the Atlantic by the closure of the Strait of Gibraltar five million years ago, and evaporated to a salty desert.
1
21,000 years ago, sea levels were 3 100,000 years ago, water from melting 120m (390ft) lower than they are today ice started to flood the continental due to water being locked up in ice caps shelves exposed during the glacial, at the height of the last glacial. leaving today’s familiar shoreline.
2
Sedimentary Basins BASINS AND OILFIELDS
Sedimentary basins are found on the continental shelves and adjacent ocean floor, but also well inland where areas were once covered with water.
AT L A
PA C I F I C
N
OCEAN
T
PA C I F I C
IC
O
KEY
OCEAN
AN
onshore sedimentary deposits
CE
Most of the world’s sedimentary rocks were laid down in water over continental shelves or in inland seas. The movements of the continents and changes in sea level have determined where this deposition occurred at particular times, and many former marine sedimentary basins are now far inland. Oil and gas deposits are found in marine sedimentary rocks, the result of animal and plant remains decomposing and then being buried and compressed. About 30 per cent of the world’s oil and gas production comes from offshore fields, but many offshore basins remain to be explored.
INDIAN OCEAN
offshore sedimentary deposits Oil and gas deposits
SOUTH
ERN OCEAN
During glacials, sea ice forms at lower altitudes than it does today. This scene may have been typical of the shores of western Europe 21,000 years ago.
INTRODUCTION
GLACIAL COAST
48
OCEAN GEOLOGY
Tectonics and the Ocean Floor THE THEORY OF PLATE TECTONICS HAS REVOLUTIONIZED
geology over the last half century, explaining many of the Earth’s physical features. Tectonic plates are huge fragments of the Earth’s lithosphere, which consists of the crust fused with the top layer of the upper mantle. They move over a more deformable layer of the mantle called the BASALT asthenosphere. Plate motion builds mountain ranges, but plateThe ocean floor is largely made of basalt, a fine-grained igneous rock tectonic processes are perhaps most clearly seen on the derived from the upper mantle. ocean floor, where most plate boundaries are found. It is a dense rock due to a high proportion of iron and magnesium. hotspot produces volcanic activity mid-ocean ridge lithospheric plate pushed away from ridge
rising mantle plume forms hotspot at surface
Recycling Ocean Crust The oldest rocks on the ocean floor are 180 million years old. This is young compared with the oldest continental rocks, which date from about 3.8 billion years ago. While the continental crust has been steadily accumulating throughout the Earth’s history, it seems the oceanic crust is created and destroyed rather quickly. It is created at the mid-ocean ridges from hot material rising in the mantle, and then spreads away from the ridges, before eventually being recycled into the mantle at subduction zones. Continental crust is always less dense and more buoyant than oceanic crust, so where they meet, it is the oceanic crust that gives way, sinking (subducting) back into the mantle.
AGE OF THE OCEAN FLOOR
The age of the ocean floor increases away from the spreading ridges where new crust is forming. The map below shows the East Pacific Rise to be the fastestspreading ridge, since it is flanked by the broadest spread of young rock (shaded red and orange).
incipient mantle plume
convection cell
oceanic lithosphere descends at subduction zone
MANTLE CONVECTION
Convection cells in the Earth’s mantle are the driving force behind plate tectonics. The cycle of hot material rising, cooling, spreading out, and sinking pushes and pulls the lithospheric plates around.
INTRODUCTION
Plate Boundaries
KEY ocean ridges at divergent plate boundaries direction of plate movement
The boundaries of a tectonic plate may be divergent, convergent, or transform. At divergent boundaries, the crust is extended, thinned, and fractured by the upwelling of hot mantle material. The crust buoys up, producing a mid-ocean ridge, and lava is extruded through a central rift valley to create new oceanic crust. Seamount volcanoes may also arise (see p.174). magma Plates collide at convergent boundaries. rises from mantle Where oceanic lithosphere meets continental lithosphere, the crust on the continental side may be compressed and thickened, resulting in mountain-building. The oceanic lithosphere sinks beneath the lighter continental lithosphere, forming an ocean trench (see p.183), and volcanic activity occurs above the descending plate. Where slabs of oceanic lithosphere converge, the oldest, most dense is subducted and an arc of volcanic islands is formed parallel to the trench. Transform boundaries arise where plates are moving past each other. No plate is created or destroyed. They can occur where segments of a divergent boundary are offset, and extensive fracture zones can result.
transform plate boundary
144
age (millions of years) 154
89
127
54.8
65
24
33.5
1.8
5
0
undated
DIVERGENT AND TRANSFORM BOUNDARIES
At divergent boundaries, parallel ridges emerge as new ocean floor spreads out either side of an ocean ridge. A transform boundary arises when sections of the ridge are offset from each other.
continent compressed, forming volcanic mountains
movement of oceanic lithosphere
oceanic trench
movement of oceanic lithosphere
movement of continental lithosphere
CONVERGENT BOUNDARIES
Ocean lithosphere is destroyed by subduction at convergent boundaries. The subducting plate carries water with it, which allows the surrounding mantle to melt, forming explosive volcanoes above.
magma forms as plate descends
oceanic lithosphere subducted beneath continental lithosphere
oceanic crust
TECTONICS AND THE OCEAN FLOOR
Earthquakes and Tsunamis
49
DISCOVERY
Earthquakes are associated with all plate boundaries, but they are particularly frequent at convergent boundaries, such as subduction zones. Stress builds up at faults in the crust until it overcomes the strength of the rock and the fault slips. When this happens, a huge amount of energy can be released in a short time. The earthquake that produced the 2011 Tohoku Tsunami in Japan released 600 million times more energy than the Hiroshima atomic bomb. A tsunami may be triggered if an earthquake results in the uplift or subsidence of part of the seafloor. The water above suddenly rises or sinks, then flows to regain equilibrium. Surface waves radiate out at 310–497 mph (500–800 kph) and can quickly cross an entire ocean basin. surface waves spread out at high speed
waves spread in opposite directions
TSUNAMI ALERTS Tsunamis can be very destructive, so systems have been established to look out for their distinctive signs and give warning of their approach. These systems use networks of seismic stations to detect earthquakes, and automated deep-sea buoys with seafloor pressure sensors to confirm whether a tsunami has been generated. The prototype buoy pictured at right is destined for seismic monitoring off the Caribbean coast of Grenada.
waves of moderate size in deep ocean
water suddenly elevated above fault
SEISMIC SEA WAVES
Tsunami waves increase in size as they encounter shallow water near the shore. They can grow from 10 ft (3 m) in the open ocean up to 100 ft (30 m) in extreme cases at the coast. waves become tall and destructive in shallow water
shockwaves spread out from the earthquake in all directions
few buildings can survive the onslaught of a large tsunami
movement along fault causes uplift of seafloor
Hotspots and Island Chains The seafloor between plate boundaries is far from featureless.Volcanic island chains are found far from any plate boundary due to the presence of hotspots (deep-seated and long-lived zones of volcanic activity) in the mantle. Some hotspots, such as the one beneath Iceland, are associated with divergent plate boundaries, while others lie in the middle of oceanic or continental plates. Chains of volcanoes often trail away from mid-ocean hotspots, with the oldest volcanoes, long extinct, now lying far away from the hotspot. These hotspot tracks are aligned along the direction of motion of the overlying plate. They change direction when the plate motion changes and may be interrupted when a new spreading ridge opens up, as it has between India and the Réunion hotspot.
seawater moves in circular motions beneath each wave as it passes
Iceland Yellowstone Azores Bermuda Canary Is. Cape Verde Is.
Hawaii Is.
Galapagos Samoa Marquesas
Hoggar
Caroline Is.
Molokini Island is the tip of an extinct volcanic crater, part of the Hawaiian–Emperor chain of islands and seamounts that stretches across the north Pacific.
Cameroon Ascension Is.
VOLCANIC ISLAND
St Helena Réunion
Easter Is. Crozet Is.
Bouvet Is.
Kerguelen Is.
HOT SPOTS
Some hot spot tracks link to areas where huge amounts of basalt flooded from the hotspot onto the surface long ago. The Tristan da Cunha hotspot is linked to flood basalts on both sides of the south Atlantic Ocean.
KEY
Plate boundaries flood basalts
convergent
hotspot tracks
transform
hotspot
divergent uncertain
INTRODUCTION
Tristan da Cunha
ERUPTION OF A SUBMARINE VOLCANO
Much of the world’s volcanic activity occurs below the ocean surface. Occasionally, an undersea volcano that has been growing for millennia reaches the sea surface and produces a dramatic eruption—as can be seen here, in an event that occurred near Hunga Ha’apai, Tonga, in the southwest Pacific, in March 2009.
OCEAN WATER IS constantly in motion, and
not simply in the form of waves. Throughout the oceans, there is a continuous circulation of seawater, both across the surface and more slowly deeper down. Several related processes play a part in causing and maintaining these ocean currents. They include solar heating of the atmosphere, prevailing winds, the effect of Earth’s rotation, and processes that affect the temperature and salinity of surface waters. The various surface currents that are generated, some warm, some cold, have profound effects on climate in many parts of the world. Oceanic processes also play a part in the periodic climatic disturbances called El Niño and La Niña, and they help generate the extreme weather phenomena known as hurricanes and typhoons.
C I R C U L ATI ON A N D C L IM AT E SPIRALING STORMS
Two cyclones—spiraling areas of low atmospheric pressure accompanied by cloud—are visible in this satellite image of part of the North Atlantic, taken in late 2006. The cyclones are moving eastward to the south of Iceland, which can be seen at top center.
54
CIRCULATION AND CLIMATE polar easterly
Ocean Winds
polar-front jet stream—narrow ribbon of strong wind at high altitude at top of front
THE PATTERN OF AIR MOVEMENT
over the oceans results from solar heating of the atmosphere and Earth’s rotation. This pattern of winds is modified by linked areas of low and high pressure (cyclones and anticyclones), which continually move over the oceans’ surface. Near coasts, additional onshore and offshore breezes are common. These are caused by differences in the capacity of sea and land to absorb heat.
The Coriolis Effect
initial direction of air movement
The atmospheric cells cause north–south air movements. These are altered by the Coriolis effect. As the Earth spins, parcels of air at different latitudes in the atmosphere have different west-to-east velocities (air at the Equator moves fastest). When they change latitude by moving to the north or south, they retain these west-to-east velocities, which differ from those of air in the AIR DEFLECTIONS In the Northern Hemisphere, latitudes they move into. Hence, the air veers to the Coriolis effect causes all air movements to be the east (in the direction deflected to the right of of Earth’s spin) when their initial direction. In moving away from the the Southern Hemisphere, Equator and to the west they veer to the left. when moving toward it.
westerlies
polar northeasterlies
westerlies
northeasterly monsoon (Nov–Mar)
northeasterly trade winds
Tropic of Cancer
Intertropical Convergence Zone
INTRODUCTION
air descends in subtropical latitudes Hadley cell air rises at equator
northeasterly trade wind southeasterly trade wind
trade winds meet at Intertropical Convergence Zone
air descends at pole
DISCOVERY
air deflected to right
air deflected to left
Ferrel cell
southwesterly wind
CIRCULATION CELLS Solar heating causes the air in Earth’s atmosphere to The atmospheric cells cycle around the globe in three sets of giant loops, produce north–south called atmospheric cells. Hadley cells are produced by airflows. These are by Earth’s warm air rising near the equator, cooling in the upper modified spin, producing winds atmosphere, and descending to the surface around that blow diagonally. subtropical latitudes (30˚N and S). Then the air moves subtropical back toward the equator. Ferrel cells are produced by jet stream air rising around subpolar latitudes (60˚N and S), cooling and falling in polar-front jet stream the subtropics, and then moving toward the poles. Polar cells are caused by air descending at the poles and moving toward the equator.
Earth’s rotation
air rises in subpolar latitudes
direction of Earth’s spin
Atmospheric Cells
initial direction of air movement
polar cell
equator
Tropic of Capricorn
SATELLITE IMAGING Ocean winds are monitored by instruments called scatterometers, such as an instrument called ASCAT on the METOP-A satellite (right). A scatterometer is a radar device that can measure both wind speed and direction.
ASCAT antenna (one of three)
Prevailing Winds The winds produced by pressure differences and modified by the Coriolis effect are called the prevailing winds. In the tropics and subtropics, the air movements toward the equator in Hadley cells are deflected to the west. These are known as trade winds. They comprise the northeasterly trades in the Northern Hemisphere, and southeasterly trades in the south. At higher latitudes, the surface winds in Ferrel cells deflect to the east, producing the westerlies. In the Southern Hemisphere, these winds blow from west to east without meeting land. Those around latitudes of 40˚S are known as the Roaring Forties. In polar regions, winds deflect to the west as they move away from the poles. These are known as polar northeasterlies and southeasterlies. KEY
prevailing warm southeasterly trade winds
westerlies
southeasterly trade winds
southeasterlies
southeasterly trade winds
westerlies
southwesterly monsoon (Apr–Oct)
prevailing cool local warm local cool
PATTERN OF WINDS
Year-round, the winds over most oceans are trades or westerlies. An exception is the northern Indian Ocean—this has a monsoon climate, in which a seasonal switch in wind direction occurs.
55 LONG-HAUL SAILING
Winds can blow with a consistent strength and direction over large areas of ocean. Consequently, on long-haul sailing trips, the same basic sail settings can often be used for days on end.
air ascends from cyclone
Pressure-system Winds
warm air rising air descends into anticyclone low pressure at center
central area of high pressure cold air sinks
air spirals around central area of low pressure
cold air flows toward area of low pressure
air moving from high to low pressure deflected by Coriolis effect to form spiral
In any area of ocean where air sinks—often at subtropical latitudes—a zone of high atmospheric pressure, or anticyclone, develops. Where warm air rises, areas of low pressure, called cyclones or depressions, occur. These often develop near the equator and subpolar latitudes. Cyclones and anticyclones create linked, circulating wind patterns, which continually move and change. In the Northern Hemisphere, there is a clockwise movement of air around an anticyclone, and a counterclockwise motion CYCLONES AND ANTICYCLONES around a cyclone. This pattern is reversed in the Air moves from an area of Southern Hemisphere. Local pressure systems can affect high pressure toward one the general pattern of prevailing winds. In particular, of low pressure, but the cyclones move swiftly over the ocean and can produce Coriolis effect modifies this, producing circular winds. rapid changes in wind strength and direction. warm air cools at high altitude
Coastal Breezes
On warm coasts, there is often a noticeable drop in temperature from midday as a cool sea breeze blows in off the water. The breeze typically reverses in the evening and at night.
DAY AND NIGHT
Land heats up faster
Local winds, called onshore and offshore than water during the day. air heats up Warm air rises over the land and rises over cool air breezes, are generated near coasts, especially in land drawn in and draws in cold air from sunny climes. Onshore breezes—sometimes the sea. At night, the land called sea breezes—develop during the day. cools more quickly, These are caused by the land heating up more reversing the airflow. quickly than the sea, as both absorb solar radiation. This occurs because the sea absorbs ONSHORE BREEZE large quantities of heat energy with only a small rise in temperature, whereas the same amount of heat energy is cold air sinks air heats up likely to cause the land temperature to rise sharply (see p.31). cool air drawn and rises As the land warms up, it heats the air above it, causing the air to rise. over ocean seaward Cooler air then blows in from the sea to take its place. In the evening, and at night, the opposite effect occurs. At nightfall, the land quickly cools down, but the sea remains warm and continues to heat the air above it. As this warm air rises, it sucks the cooler air off the land, and so generates an offshore breeze. This is sometimes called a “land breeze.” OFFSHORE BREEZE
INTRODUCTION
BREEZY COAST
cold air sinks
TRIMMING THE SAILS
A crew sets their sails as they set off on the Sydney-to-Hobart yacht race. The course crosses the often stormy Bass Strait between Australia and Tasmania.
57
Ocean Yacht Racing
Racing Around the Globe Three of the most famous ocean yacht races are global circumnavigations. They go “round the right way” in the Southern Ocean (west to east, the same direction as the prevailing winds and currents). The Volvo Ocean Race, held every three years, is a team event and includes stops. The Velux 5 Oceans Race and Vendée Globe are Vendée Globe single-handed races, each held every Velux 5 Oceans four years. The Velux 5 is in stages, 2010–11 Volvo 2014–15 whereas the Vendée Globe is nonstop.
ATLANTIC OCEAN
Abu Dhabi
MULTIHULL RACING
TRIMARAN Most ocean yacht races are for monohull yachts, but a few are open to multihulls (catamarans and trimarans), while some are for multihulls only. Multihulls are faster than monohulls, and although easier to capsize, they stay afloat even when severely damaged. Here, the trimaran Foncia is sailing on just one of its three hulls during the 2005 Grand Prix de Fécamp, off Normandy, France.
YACHT CREWS
ALL HANDS ON DECK The crews for some races are large— in the Clipper Round the World Yacht Race there are typically as many as 18 on board at any one time. For this race, the crew, who are recently trained amateurs, participate in all duties on the yacht, including trimming sails, navigating, helming, cooking, and so on.
TURNED TURTLE In 1997, the lone British yachtsman Tony Bullimore capsized in the Southern Ocean and was trapped for five days in his upturned yacht before rescuers arrived.
HANGING ON The crew of the yacht Astra (shown here) risk being swept overboard, the greatest disaster that can befall a sailor. To assist recovery in this eventuality, crew members usually wear radio beacons. It is also usual to be attached to the boat via a safety harness in anything other than calm daylight conditions.
PACIFIC OCEAN Sanya
Itajai
INDIAN OCEAN
Cape Town SOUTHERN OCEAN
Auckland Wellington
RISKY BUSINESS
PACIFIC OCEAN Recife Salvador
CONTROL CENTER Technology is all-important in modern racing, including the use of electronic charts and global positioning systems. Here, Frenchman Marc Thiercelin prepares for the Vendée Globe 2005–06 in the control center on his yacht Pro-Form.
INTRODUCTION
Göthenburg Les Sables-d’Olonne Lorient Newport La Rochelle Charleston Lisbon Alicante
EQUIPPED TO WIN
HIGH-TECH AIDS
Ocean yacht racing is the sport of competitive sailing, held over long distances and in open water. These races range from short but robust challenges lasting a few days, such as the famous Fastnet Race off southwest England and the annual Sydney-to-Hobart race, to long, multi-stage, around-the-world races, which can involve up to 6 months at sea. Some races are multi-handers, with crews that can be as large as 20 or more per yacht; others are single-handers. The participants are typically highly experienced sailors.In multi-handed races, there will be a skipper/tactician, a navigator, and general crew whose responsibilities include sail changing and trimming. Solo racers have to do everything themselves. One race, the Clipper Round the World Yacht Race, is unusual in that its crew consists of amateurs, some with little previous sailing experience, who have paid to take part under the leadership of a professional skipper. To make races as equitable as possible, usually competing boats are identical or a handicapping system is used to adjust the times of different classes of boats. The use of computer technology is paramount in modern racing. Navigation is electronically assisted, and computers are employed to monitor and help optimize boat performance.Vast amounts of weather data are downloaded via the Internet during a race. An important skill is to be able to interpret this data, so as to know, for example, where the most wind is likely to be in the area ahead. Otherwise, doing well in a race is mainly down to tactics and seamanship— for example, knowing how to get the best out of a boat in both strong and light winds, or judging when best to tack (change course when sailing upwind).
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CIRCULATION AND CLIMATE
Surface Currents FLOWING FOR ENORMOUS DISTANCES
within the upper regions of the oceans are various wind-driven currents. Many join to produce large circular fluxes of water, called gyres, around the surfaces of the main ocean basins. Surface currents affect only about 10 percent of ocean water, but they are important to the world’s climate (see p.66), because their overall effect is to transfer huge amounts of heat energy from the tropics to cooler parts of the globe. They also impact shipping and the world’s fishing industries. direction of
Coriolis deflection wind
frictional wind drag
Wind on Water
resultant direction of water motion
When wind blows over the sea, it causes the upper ocean to move, creating a current. However, the water does not move in the same direction as the wind. Instead, it moves off at an angle—to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This phenomenon was first explained in 1902 by a Swedish scientist, Walfrid Ekman, using a model of the effect of wind on water now called the Ekman spiral. The model assumes that the movement of water in each layer of the upper ocean is produced by a combination of frictional drag from the layer above (or, in the top layer, from wind drag) and the Coriolis effect (see p.54). The model predicts that, overall, a mass of water will be pushed at right angles to the wind direction, an effect known as Ekman transport. N. Atlantic Drift
Labrador N. Equatorial
E. Greenland
Gulf Stream
drag imparted from layer above direction of water motion
water motion in this layer
EKMAN SPIRAL Canary
Somali
Agulhas Oyashio
Alaska
Kuroshio N. Pacific California N. Equatorial
MAIN CURRENTS
Equatorial Counter
This map shows all of the world’s main surface currents, both warm and cold.
S. Equatorial E. Australia W. Australia
Antarctic Circumpolar Peru
S. Equatorial
Benguela
S. Equatorial
Ocean Gyres
INTRODUCTION
north Pacific gyre
The combination of prevailing winds (see p.54) and Ekman transport produces large-scale, circular systems of currents known as gyres. All together there are five ocean gyres—two in each of the Atlantic and Pacific oceans and one in the Indian Ocean. Each gyre westerly winds consists of several named currents. Thus, the gyre in the north Pacific is made up of the Kuroshio northeast current in the west, the California current trade winds in the east, and two other linked currents. Water tends to accumulate at the center of these gyres—producing shallow equator “mounds” in the ocean. southeast trade winds south Pacific gyre
westerly winds
direction of gyre direction of wind
warm current cold current
Brazil
GYRE CREATION
In the north Pacific, the combination of westerly and trade winds, always pushing water to the right (by Ekman transport) produces a clockwise gyre. In the south Pacific, where winds push water to the left, a counterclockwise gyre is created.
drag
The direction of motion in each water layer results from a combination of the drag from the layer above and a deflection caused by the Coriolis effect. This diagram shows the Ekman spiral in the Northern Hemisphere. In the Southern Hemisphere, deflection is to the left of wind direction.
SURFACE CURRENTS
59
PEOPLE
BENJAMIN FRANKLIN The American statesman and inventor Benjamin Franklin (1706–90) made one of the earliest studies of an ocean current, publishing a map of the Gulf Stream’s course. He became interested in it after the British postal authorities asked him why American postal ships crossed the Atlantic faster than English ships. The answer was that American ships were utilizing an eastward extension of the Gulf Stream.
Boundary Currents The currents at the edges of gyres are called boundary currents. Those on the western side of gyres are strong, narrow, and warm—they move heat energy away from the equator. Examples of these currents are the Gulf Stream and the Brazil Current in the southwestern Atlantic. Eastern boundary currents are weaker, broader cold currents that move water back toward the tropics. Examples are the Benguela Current off southwest Africa and the California Current. At the gyre boundaries close to the equator are warm, west-flowing equatorial currents. Other currents feed into or out of the main gyres. These include, for example, the warm North Atlantic Drift, an offshoot of the Gulf Stream, and cold currents that bring water down from the Arctic, such as the Oyashio and East Greenland currents. WARM CURRENT
Satellite devices can detect phytoplankton levels in the water, which can be related to temperature. Here, yellow and red indicate high levels of plankton and the warm Brazil Current.
COLD CURRENT
In this satellite view, sea ice is visible flowing past the Kamchatka Peninsula in the cold Oyashio Current. Eddies within the current have produced spiral patterns in the sea ice.
Meeting of Currents In a few areas, warm and cold currents meet and interact. Examples include the meeting of the warm Gulf Stream with the cold Labrador Current off the eastern seaboard of the US and Canada, and the meeting of the cold Oyashio Current with the warm Kuroshio Current to the north of Japan. At these confluences, the denser water in the cold current dives beneath the water in the warm current, usually producing some turbulence. This can trigger an upward flow of nutrient-rich waters from the sea floor, encouraging the growth of plankton, and producing good feeding grounds for fish, sea birds, and mammals. SEA SMOKE
OPPOSING CURRENTS
INTRODUCTION
The warm Brazil Current on the left, and the colder Falklands Current on the right, each carry differently colored populations of plankton.
Dolphins cavort amid steep waves. The “sea smoke” is created when water vapor is added to cold air drifting across the boundary between cold and warm currents.
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CIRCULATION AND CLIMATE
Underwater Circulation
The most important causes of downwelling are thermohaline NORTH ATLANTIC DOWNWELLING ZONES processes (“thermo” means heat, and “haline” means salt), which At the important downwelling alter either the temperature or salinity of seawater. For example, sites shown here, warm where warm, salty water is carried by a surface current into the surface water meets colder Arctic Ocean, it rapidly cools when it meets colder, less salty, Arctic water, loses heat, polar water. As it cools, its density increases, and it sinks down. become denser, and sinks. Downwelling also occurs on some coasts. For example, winds blowing toward the equator on the western side of oceans push east-facing coast (Northern seawater toward land by Ekman transport (see p.58). Hemisphere) As it reaches the coast, it is forced down. Finally, downwelling also occurs beneath the NORTH mounds of water that accumulate in the middle of anticyclones (see p.55) and ocean gyres (see p.58). COASTAL DOWNWELLING
A wind blowing toward the Equator on the western side of an ocean, as here (left), pushes seawater toward the shore, where it sinks. wind blowing toward the equator
water sinks near coast
water pushed toward shore due to Ekman transport
water level is raised at center of anticyclone
KEY
Upwelling can occur in various situations, some of which are simply the reverse of the conditions that cause downwelling. For instance, winds blowing toward the equator on the eastern sides of oceans push seawater away from land by Ekman transport, so deeper water must upwell near the coast to replace it. Water rises toward the surface in the center of cyclones (the opposite of anticyclones, see p.55), and will also rise where surface waters tend to be pushed apart at boundaries between ocean gyres—for example, in some equatorial parts of the Pacific and Atlantic. Some seawater upwells to replace sinking, denser, water. An example occurs around Antarctica, where upwelling replaces superdense, cold, salty water forming and sinking under developing sea ice.
A
ic rct
cle cir
ICELAND
NORTH ATLANTIC OCEAN downwelling warm surface current loss of heat energy cold surface current downwelling zone
winds flow clockwise in Northern Hemisphere (counterclockwise in Southern Hemisphere)
water sinks due to effects of gravity
Upwelling
NLAND
Downwelling
Baffin Bay
A N A D C A
circulate deep below the surface. Subsurface currents are complex. Some are vertical, moving water upward and downward to and from the surface, processes called upwelling and downwelling. Surface and subsurface currents are all linked in a global pattern of deep-water circulation.
GREE
THE WATERS THAT MAKE UP EARTH’S OCEANS
accumulation of water at center Ekman transport pushes water toward center of anticyclone
DOWNWELLING IN AN ANTICYCLONE
In an anticyclone, the circular system of winds can push water into a central mound, where it sinks. west-facing coast (Northern Hemisphere)
water moves away from shore as a result of Ekman transport
wind blowing toward the equator NORTH
COASTAL UPWELLING
INTRODUCTION
water moves upward to replace the water moving offshore at the surface
A wind blowing toward the equator on the eastern side of an ocean, as here, pushes seawater away from the shore, causing upwelling near the coast.
PLANKTON-HARVESTER Where upwelling occurs, it brings large amounts of nutrients up from the sea floor. These encourage the growth of plankton, attracting planktongrazers such as this manta ray as well as smaller fish, whales, and other marine life.
UNDERWATER CIRCULATION
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Deep-water Circulation downwelling of cold, salty water in north Atlantic
cold, dense water moves at depth through Atlantic
warm surface flow in South Equatorial Current
diffuse upwelling in Indian Ocean
Seawater circulates slowly through the deeper parts of the oceans, driven by water sinking in major downwelling zones, such as in the north Atlantic. Any specific mass of deep water has, at some time, sunk in one of these zones. Once it sinks, its properties, such as its salinity, remain stable for long periods—thus, every mass of deep water contains a “memory” of where it originally sank. By analyzing seawater samples from various parts of the deep oceans, it is possible to piece together the general pattern of deep-water flow. The indications are that there is a large-scale circulation involving all the oceans, called the global conveyor. A specific mass of seawater takes about 1,000 years to complete a lap of this circuit. diffuse upwelling in north Pacific Ocean
warm surface flow of North Equatorial Current in central Pacific
Atlantic water is joined here by more cold water formed near Antarctica
warm flow of Equatorial surface current through Indonesian archipelago
DISCOVERY
SEAL AID This deep-diving elephant seal is helping to gather information about underwater circulation in the south Atlantic. A measuring device—attached to its head with glue that sloughs off when the animal molts—collects data about temperature and salinity at varying depths. The information gained may also help to conserve elephant seal populations.
THE GLOBAL CONVEYOR
The conveyor starts with cold, salty water sinking in the north Atlantic. Moving south at depth, it flows around Antarctica, branching into the Indian and Pacific oceans, and returns to the surface by mixing with warmer waters above. Finally, warm surface currents return it to the Atlantic.
combined mass of cold water moves slowly around Antarctica, at depth
cold, dense water flows north at depth into the Pacific Ocean
Circulation Cells
INTRODUCTION
One type of circulation that affects only the upper 70 ft (20 m) of the ocean, but is more complex than either a simple horizontal or vertical flow of water, is known as Langmuir circulation. This is wind-driven and consists of rows of long, cylinder-shaped cells of water, aligned in the direction in which the wind is blowing and each rotating in the opposite direction from its neighbor—alternate cells rotate clockwise and counterclockwise. Each cell is about 30–160 ft (10–50 m) wide and can be hundreds of yards long. On the sea surface, the areas between adjacent cells where seawater converges are visible as long white streaks of foam, or congregations of seaweed, called windrows. The whole pattern of circulation LANGMUIR WINDROWS long streaks of foam on the was first explained in 1938 by an American These sea surface are the windrows of chemist named Irving Langmuir, after he Langmuir circulation cells. The crossed the Atlantic in an ocean liner. distance between windrow lines increases with the wind speed. It was subsequently named in his honor.
DRIVING THE ATLANTIC CONVEYOR
Sea-ice formation on the margins of the Atlantic and Arctic helps to drive the Atlantic Conveyor. Only the freshwater component is incorporated in the ice, leaving dense, salty water that sinks to the ocean floor.
Shutting Down the Atlantic Conveyor CAUSES AND EFFECTS OF SHUTDOWN
At present, warm surface water moving north from the equator replaces cold water that sinks in the north Atlantic. Winds flowing over the warm ocean absorb heat and transfer it to western Europe. An increase of fresh water in the far northern Atlantic could mean that cold water no longer sinks there, shutting down the system and chilling Europe.
CHANGES IN ARCTIC ICE COVER There is ample evidence from satellite surveys that the extent of summer sea ice in the Arctic Ocean is diminishing rapidly at a rate of about 14 percent per decade. Should the Atlantic Conveyor shut down, however, this trend would reverse—Arctic seas close to the Atlantic, such as the Greenland Sea and Barents Sea, would become iced over all year.
ARCTIC MARINE LIFE Any shutdown in the North Atlantic Drift—the extension of the Gulf Stream that brings warm water to northwestern Europe—could have a major effect on life in Arctic areas adjacent to the north Atlantic. Changes in the currents would interfere with plankton production, affecting the whole food chain. The likely drop in temperature would also drive out some species of fish and invertebrates, which include crabs, starfish, and sea urchins.
LIFE SUPPORT FOR THE OCEAN FLOOR INFLOWS
Changes in the Atlantic Conveyor
ARCTIC SEA ICE
Global warming, it has been suggested, might have
the paradoxical long-term effect of lowering temperatures in Europe. The basis on which this could happen would be a shutdown of the Atlantic Conveyor – a system of currents that, at present, keeps western Europe warm. Part of a worldwide pattern of connected currents, the Atlantic Conveyor has two main components. The first is a flow of warm surface water into the northeastern Atlantic in the North Atlantic Drift – an extension of the Gulf Stream. The second component is the sinking of cold, salty water in the far north and the subsequent deep-ocean flow of this water back toward the equator. The conveyor might shut down if the Arctic seas are flooded with fresh water as a result of melting sea-ice and increased river run-off caused by global warming. Since fresh water is less dense than salt water, surface waters in these regions would become less likely to sink – risking disrupting the conveyor. If this were to occur, Europe’s average temperature could fall. How likely is this to happen? Computer models suggest that the current increase in the flow of fresh water in the Arctic is not high enough to shut down the conveyor. The models also suggest that the flows will not reach high enough levels for at least a century. Although a weakening of the conveyor could occur over the medium term, the models suggest that the overall outlook in this time frame could still be warming, rather than cooling, over Europe.
63
THE MACKENZIE RIVER DELTA Increased flow in Arctic rivers such as the Mackenzie—caused by the melting of glaciers and permafrost—might contribute to a shutdown of the Atlantic Conveyor by flooding fresh water into the Arctic Ocean.
THE SCILLY ISLES Situated 30 miles (50 km) off the southwest tip of Great Britain, the Scilly Isles sit directly in the path of the North Atlantic Drift. As a result, they enjoy a subtropical climate and are a haven for plants from all over the world. If the Atlantic Conveyor ground to a halt, gardens such as these would become desolate as average temperatures plunged to about 9°F (5°C).
PRESENT CIRCULATION cold water moves south over ocean floor
END OF THE ATLANTIC CONVEYOR lid of less salty water over denser, saltier water winds absorb less heat from ocean, and transfer less warmth to Europe
warm Gulf Stream no longer flows into north Atlantic
INTRODUCTION
warm water flows north from equator, transferring heat northward
THE NORTH ATLANTIC DRIFT
winds absorb heat from ocean and transfer it to Europe
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CIRCULATION AND CLIMATE
The Global Water Cycle
GLOBAL WATER FLOW
Water enters the atmosphere mainly as a result of evaporation from the oceans and transpiration by plants. It condenses to form clouds and falls as rain and snow. On land, water moves downhill in rivers and glaciers. It soaks into the soil and rocks, and is stored in lakes and wetlands.
THE WORLD’S OCEANS DO NOT FORM
a self-contained system but continually exchange water with the atmosphere and landmasses through evaporation, cloud formation, precipitation, wind transport, and river flow. This complex of interconnected processes, which is ultimately driven by heat from the Sun, is called the global water cycle or hydrologic cycle. The cycle is made up of many smaller cycles, such as the formation and melting of sea-ice.
snow falls when moisture in cold air freezes at high altitude snow and ice accumulate on high mountains
clouds form as rising air cools and the water vapor it holds condenses water returns to land as rain when moisturecarrying clouds cool
winds blow moisture-laden clouds inland
evaporation of water from ocean, driven by solar heating
in summer, snow and ice melt, releasing fresh water
release of water by plants through transpiration
evaporation of moisture from the ground as a result of solar heating
rivers steadily transport water towards the ocean
eventually, downhill flow means rivers flow into the oceans
ocean water is salty because it contains dissolved nutrients
below a line known as the water table, the rock is saturated with water
water collects in hollows in the ground, forming freshwater lakes
water can flow downhill underground as well as above ground
cracks and holes in the rocks allow them to be filled with water
atmosphere and other 0.09%
INTRODUCTION
Earth’s Water Reservoirs
PLAYERS IN THE CYCLE
The sea, ice, mountains, and clouds all play a part in the global water cycle. This coastal scene is near Port Lockeroy in Antarctica.
Just under one-third of a billion cubic miles (1.4 billion cubic kilometers) of water exists on Earth. About 97 percent of this water is stored in the oceans as a component of salt water. The rest is fresh water. Of this, more than two-thirds is in the form of ice, locked up in the vast ice-sheets that cover Antarctica and most of Greenland, and in icebergs and sea-ice. Much of the rest is groundwater—contained in underground rocks—while a tiny amount (less than 1 part in 2,000) is water vapor in the atmosphere. Fresh liquid water on the Earth’s land surface, in lakes, wetlands, and rivers, makes up just 0.3 percent of all the world’s fresh water, or 0.02 percent of the total water. The Earth’s different water reservoirs have not always had the same relative sizes that they have today. For instance, during the ice ages, a higher proportion was locked up in ice, with less in the oceans.
ocean water 97%
fresh water 3.5%
surface fresh water 0.3% groundwater 30.1% rivers 2%
EARTH’S WATER
wetlands 11%
ice 69.5% lakes 87%
FRESH WATER SURFACE FRESH WATER
RELATIVE SIZES
Earth’s ocean water (the bulk of the rear cylinder, above) hugely exceeds its reservoirs of fresh water, and the relative proportion of fresh water found on the land surface is tiny.
65
Ocean Evaporation and Precipitation A total of 104,000 cubic miles (434,000 cubic kilometers) of water evaporates from the oceans per year. Of this, 95,500 cubic miles (398,000 cubic kilometers) falls back into the sea as precipitation (rain, snow, sleet, and hail). The remainder is carried onto land as clouds and moisture. Evaporation and precipitation are not evenly spread over the surface of the oceans. Evaporation rates are greatest in the tropics and lowest near the poles. High rates of precipitation occur near the equator and in bands between the latitudes of 45º and 70º in both hemispheres. Drier regions are found on the eastern sides of the oceans between the latitudes of approximately 15º and 40º. PEOPLE
SENECA THE YOUNGER In his book Natural Questions, the Roman statesman, dramatist, and philosopher Seneca the Younger (4 bc–ad 65) pondered why ocean levels remain stable despite the continuous input of water from rivers and rain. He argued there must be mechanisms by which water is returned from the sea to the air and land and proposed an early version of the hydrologic cycle to explain this.
Freshwater Inflow The 8,500 cubic miles (36,000 cubic kilometers) of water lost from the oceans each year by evaporation and transport onto land is balanced by an equal amount returned from land in runoff. Just 20 rivers, including the Amazon and some large Siberian rivers, account for over 40 percent of all input into the oceans. Inflows from the different river systems change over time as they are affected by human activity and climate change. For instance, global warming appears to have increased the flow from Siberian rivers into the Arctic Ocean, as water frozen in the tundra melts. These inflows lower the salinity of Arctic waters and may influence global patterns of ocean circulation (see p.63).
SIBERIAN RIVER AMUR FLOODING
Climate change is thought to have contributed to severe flooding of the Amur River in Northeast Asia in recent years (below). A color-enhanced satellite image (left) shows the engorged river in black; green areas are plant-covered land.
EQUATORIAL RAINSTORM
In some areas near the equator, as here in the tropical Pacific, annual rainfall is over 120 in (300 cm), compared to under 4 in (10 cm) in the driest ocean areas.
The Sea-ice Cycle
SEA-ICE FORMING
As sea-ice forms, it releases heat to the atmosphere and increases the saltiness of the surrounding water (by rejecting salt). These processes affect climate and the circulation of seawater.
INTRODUCTION
In addition to the overall global water cycle, there is a local seasonal cycle in the amount of water locked up as sea-ice. In the polar oceans, the extent of sea-ice increases in winter and decreases in summer. This has important climatic consequences, because sea-ice formation releases, and its melting absorbs, latent heat to and from the atmosphere; and because the presence or absence of sea-ice modifies heat exchange between the oceans and atmosphere. In winter, sea-ice insulates the relatively warm polar oceans from the much colder air above, thus reducing heat loss. However, especially when covered with snow, sea-ice also has a high reflectivity (albedo) and reduces the absorption of solar radiation at the surface. Overall, the sea-ice cycle is thought to help stabilize air and sea temperatures in polar oceans. Also, because it affects surface salinity, sea-ice formation helps drive large-scale circulation of water through the world’s oceans (see p.61).
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CIRCULATION AND CLIMATE
Oceans and Climate THE OCEANS HAVE A PROFOUND INFLUENCE
on the world’s climate, most strikingly in the way they absorb solar energy and redistribute it around the world in warm surface currents. Cold currents also produce local climatic effects, while alterations in currents are associated with climatic fluctuations such as SOLAR HEATING surface layers of the oceans El Niño (see p.68). The future behavior of the oceans is crucial The absorb about half the solar to the future course of climate change, as they are an important energy that reaches Earth. Currents move this from the store for carbon dioxide, the principal greenhouse gas. equator toward the poles at a rate of several billion megawatts.
Warm Currents Five or six major surface currents (see p.58) carry heat away from the tropics and subtropics toward the poles, giving some temperate regions a warmer climate than they would otherwise enjoy. A prime example is the effect of the warm Gulf Stream and its extension, the North Atlantic Drift, on Europe. The North Atlantic Drift carries heat originally absorbed in the Caribbean Sea and Gulf of Mexico across the Atlantic, where it is released into the atmosphere close to the shores of France, the British Isles, Norway, Iceland, and other parts of northwestern Europe. As the prevailing westerly winds blow this warmed air over land, these countries benefit from a milder climate than equivalent regions at similar, or even lower latitudes, on the western side of the Atlantic. For example, winter temperatures are typically BALMY BEACHFRONT higher in Reykjavik, the capital of Iceland, than in New York. Similarly, in the northwest Penzance, in southwest England, has a mild climate that supports Pacific, the Kuroshio Current warms the subtropical vegetation—the southern part of Japan, while in the extreme effect of the North Atlantic Drift southwest Pacific, the East Australian Current is to raise temperatures here by about 9˚F (5˚C). gives Tasmania a relatively mild climate.
INTRODUCTION
Cold Currents
STRAYING NORTH
In some instances, the climatic effect of cold currents is simply to produce a cooler climate than would otherwise be the case. For instance, the west coast of the US is cooled in summer by the cold California Current. Cold currents also affect patterns of rainfall and fog formation. In general, the various cold currents flowing toward the equator on the eastern sides of oceans—combined with upwellings of cold water from the depths in these regions— cool the air, reduce evaporative losses of water from the ocean, and cause downdrafts of drier air from higher in the atmosphere. Although clouds and fog often develop over the ocean in these areas (as what little moisture there is condenses over the cold water), these quickly disperse once the air moves over land.Thus, cold currents contribute to the development of deserts on land bordering the eastern sides of oceans, such as the Namib desert in southwestern Africa.
COAST OF NORTHERN CHILE
The cold Peru Current flows along the coast of northern Chile. It encourages the development of clouds and fog over the sea (visible above left in the satellite image) but also contributes to the extreme aridity of the coastal strip (left).
Most penguins live in Antarctica but, somewhat surprisingly, the world’s most northerly-living penguins inhabit the Galápagos Islands, on the equator. The islands have a cool climate— sea-surface temperatures in most years average 9˚F (5˚C) less than typical temperatures in the tropics, due to the cold Peru Current that flows up the west coast of South America.
OCEANS AND CLIMATE FORAMINIFERAN SHELL
Carbon in the Oceans
The oceans contain Earth’s largest store of carbon dioxide (CO2)—the main greenhouse gas implicated in global warming. Huge amounts of carbon are held in the oceans, some in the form of CO2 and related substances that readily convert to CO2, and some in living organisms. The oceanic CO2 is in balance with the atmospheric content of the same gas. For many years, the oceans have been alkaline, and acted as an important store for the excess CO2 released by human activity. Biological and chemical processes turn some of this CO2 into the calcium carbonate shells and skeletons of organisms, other organic matter, and carbonate sediments. However, the increasing CO2 concentration is beginning to acidify the oceans, threatening shell and skeleton formation in marine organisms, as acid tends to dissolve carbonates. Further, some scientists fear that the rate at which the oceans can continue to absorb CO2 will soon slow down, further aggravating global warming. CO2 released by plant respiration
CO2 absorbed by photosynthesis
CARBON CONVERSION
CO2 released from burning fossil fuels (right), after absorption into the oceans, can eventually end up in the shells of marine organisms in the form of carbonate. CO2 released by volcanic eruption
CO2 released by fossil-fuel burning
CO2 in rain weathers limestone
CO2 released by fossil-fuel burning
CO2 absorbed by photosynthesis by phytoplankton
CO2 released by land animal respiration
METHANE HYDRATE DEPOSIT
This substance is found as a solid on some areas of sea floor. There are concerns that ocean warming could release this into the atmosphere as methane gas, which traps more heat than CO2.
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CO2 released by marine animal respiration
CO2 removed from storage by coal mining carbon from plant and animal remains stored in form of coal deposits
CO2 released by phytoplankton respiration
CARBON SOURCES AND STORES
At present, more CO2 is added to than subtracted from the atmosphere. Some of the excess is absorbed by the oceans, where some is held in solution and some incorporated into living organisms and sediments.
carbon released by decomposition of marine organisms
carbon released by decomposing phytoplankton
oil and gas
carbon in sediment turns into oil and gas
carbonate in sediment turns into limestone
The climate of San Francisco is influenced by exceptionally cold water, produced by upwelling, off the California coast. Fog is produced as westerly winds blow moist air over this cold water.
INTRODUCTION
GOLDEN GATE FOG
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CIRCULATION AND CLIMATE
El Niño and La Niña EL NIÑO AND LA NIÑA ARE LARGE
climatic disturbances caused by abnormalities in the pattern of sea surface temperature, ocean currents, and pressure systems. They are in the tropical Pacific Ocean. These disturbances have important repercussions for weather throughout the Pacific and beyond. Most scientists regard El Niño and La Niña as extreme phases of a complex global weather phenomenon called the El Niño–Southern Oscillation (ENSO).
descending air associated with high pressure and dry conditions
southeast trade winds
low-pressure system in western Pacific with rising warm, moist air and associated heavy rainfall
El Niño Events The Spanish term el niño means “the little boy” or “Christ child.” It originally denoted a warm current that was occasionally noticed around Christmas off Peru. Later it was restricted to unusually strong rises in temperature in the waters of the eastern Pacific, with a reduction in the upwelling of nutrient-rich waters that normally occur there. It is now used to mean a much wider shift in ocean and atmospheric conditions that affects the whole globe. El Niño events typically last from 12 to 18 months and occur cyclically, although somewhat unpredictably. On average, they occur about 30 times per century, with intervals that are sometimes as short as three years and sometimes as long as 10 years. Their underlying cause is not understood. TEMPERATURE PATTERNS
These satellite-generated images of the Pacific compare surface temperature patterns. Red and white indicate warm water; green and blue denote cooler water.
pool of warm water South Equatorial Current
upwelling of cold, nutrient-rich water
NORMAL PATTERN southeast trade winds reverse or weaken
descending air and high pressure brings warm, dry weather
A low-pressure system in the western Pacific draws southeast trade winds across from a high-pressure system over South America. These winds drive the South Equatorial Current, which maintains a pool of warm surface water in the western Pacific. low pressure and rising warm, moist air associated with heavy rainfall
warm water flows eastward, accumulating off South America
EL NIÑO PATTERN JANUARY 2011 (NORMAL)
upwelling blocked by warm water near surface
During an El Niño event, the pressure systems that normally develop in the Pacific, and the southeast trade winds, weaken or reverse. The pool of warm surface water extends from the western Pacific into the central and eastern Pacific.
DECEMBER 2009 (EL NIÑO)
INTRODUCTION
Effects of El Niño
GIANT WAVES
An El Niño event causes wetter-than-normal conditions, and floods, in countries on the western side of South America, particularly Ecuador, Peru, and Bolivia. These conditions may also extend to the southeastern United States. In other parts of the world, it causes drier conditions. Drought and forest fires become more common in the western Pacific, particularly in Indonesia and parts of Australia, but also in East Africa and northern Brazil. The warmer waters in the eastern Pacific cause a reduction in the Peru Current and reduced upwelling near the coast of South America. This reduces the level of nutrients in the seawater, which has a negative impact on fish stocks. Other effects include a quieter Atlantic hurricane season and an increase in the extent of sea ice around Antarctica. Japan, western Canada, and the western US typically experience more storms and warmer weather than normal. width of rings directly related to amount of growth 1746–47 El Niño ring
EVIDENCE OF AN HISTORICAL EL NIÑO
Increased tree growth can be linked to high rainfall that occurred during historic El Niño events. One of the rings in this sample has been linked to an El Niño in 1746–47.
During an El Niño event, storms become more frequent and violent in the central Pacific. These storms can produce gigantic waves, up to 33 ft (10 m) high, in Hawaii, as here on the island of Oahu.
CORAL BLEACHING
This small circular coral reef has suffered severe bleaching (whitening). El Niño events are often associated with bleaching caused by unusually high sea surface temperatures.
EL NIÑO AND LA NIÑA DISCOVERY
MONITORING The tropical Pacific is regularly monitored for temperature changes. The main monitoring methods are the use of satellites, which measure sea temperatures indirectly from slight variations in the shape of the ocean surface, and an array of instrumented weather buoys. INSTRUMENTED BUOY
Buoys such as this one, strung in an array across the equatorial Pacific, are used to make regular measurements of water temperature at varying depths.
La Niña Events La niña is Spanish for “the little girl.” A La Niña event is the reverse of an El Niño event. It is characterized by unusually cold ocean temperatures in the eastern and central equatorial Pacific, and by stronger winds and warmer seas to the north of Australia. La Niña conditions frequently, but not always, follow closely on an El Niño. Like El Niño, La Niña causes increased rainfall in some world regions and drought in others. India, Southeast Asia, and eastern Australia are lashed by rains, but southwestern US generally experiences higher temperatures and low rainfall. Meanwhile, northwestern states of the US pool of warm water positioned experience colder, snowier winters. farther west than La Niña is also associated with an normal increase in Atlantic hurricane activity. Overall, the effects of a La Niña event often tend to be strongest during Northern Hemisphere winters.
low-pressure system, positioned farther west than normal
South Equatorial Current
southeast trade winds
69
descending air associated with dry conditions and high pressure
upwelling of cold, nutrientrich water sea surface cooler than normal in eastern Pacific
LA NIÑA PATTERN
DROUGHT CONDITIONS
This water reservoir in Texas completely dried up during a severe La Niña-related drought that affected the southwestern US in 2011.
During a La Niña event, the area of low pressure in the western Pacific is farther west than normal, and the pool of warm surface water is also pushed west. Unusually cold surface temperatures develop in the eastern Pacific as the cold Peru Current strengthens off South America.
Teams of youths work to shepherd pedestrians across a highway in Peru flooded by heavy El Niño rains in 1998. This El Niño ravaged Peru, causing 250,000 people to abandon their homes.
INTRODUCTION
PERUVIAN FLOOD
70
CIRCULATION AND CLIMATE
Hurricanes and Typhoons HURRICANES AND TYPHOONS ARE TERMS USED IN
different parts of the world for very similar weather phenomena. They are characterized by violent winds moving in a circular pattern over the ocean, dense bands of clouds, and rainfall. In the Atlantic they are known as hurricanes; those in the west Pacific are called typhoons. Similar phenomena elsewhere are called severe storms or cyclones. They start as a low-pressure system (depression) over warm oceans in the tropics, between latitudes 5° and 20°, and occur mainly in late summer. A R C T I C
DISTRIBUTION
Severe tropical cyclones start as depressions over warm oceans in the tropics. They move across the ocean surface for several days, causing huge damage on reaching land. Their paths are shown in the map above.
O C E A N
A
T
L
PAC I F I C
A
N
O C E A N
Development All tropical cyclones develop from the effects of the Sun warming the surface of a broad area of ocean and the air above it. This heating causes masses of warm, moist air to rise, creating a region of low pressure at the surface, and dense clouds above it. The low pressure sucks in more air, which spirals to the center, creating a circular wind system. As it grows stronger, becoming THREE DEVELOPMENT STAGES: HURRICANE SANDY, OCTOBER 2012 a tropical storm, it is pushed westward by the prevailing trade winds. In the Atlantic, a storm attains hurricane status once its winds exceed 74 mph (119 kph). Eventually, most of these violent storms move away from the equator— that is, to the north in the Northern Hemisphere. When one reaches land, it begins to 1 On October 23 a swirling mass of 2 By October 26, the cyclone has lose energy, as it is no longer warm, moist air rises over a tropical a spiral form, with a dense central fed by heat from the ocean. area of ocean, condensing into clouds. nucleus of clouds.
INTRODUCTION
cap of cirrus clouds over cumulonimbus that forms bulk of clouds
eyewall is a ring of destructive thunderstorms and rainbands around the eye
eye of the hurricane is a calm, cloud-free area of sinking air and light winds
high-level winds spiral outward
ascending warm, moist air created by solar heating of the ocean surface
INDIAN OCEAN
O C E A N
H E R N S O U T
KEY hurricanes severe cyclones typhoons
3
At full hurricane status on October 28, the cyclone has compacted and developed a clear central “eye.”
Structure A fully developed typhoon or hurricane is usually 185–370 miles (300–600 km) in diameter and 6–9 miles (10–15 km) high. At its center is a calm region of low atmospheric pressure, called the eye. Within the rest of the cyclone, winds spiral in an counterclockwise direction in the Northern Hemisphere and clockwise in the Southern Hemisphere (the difference is due to the Coriolis effect, see p. 54). Within an area surrounding the eye, called the eyewall, the air spins upward, forming dense clouds. The eye stays calm because the winds that spiral in toward it never reach the center. Radiating out from the eye and eyewall are well-defined bands of clouds, called rainbands.
HURRICANE STRUCTURE
sea surface rises at center of hurricane low pressure at water’s surface creates warm winds; these increase in speed toward the eye
OCEAN
N CEA C O TI
P A C I F I C
spiral rainbands can extend for hundreds of miles from the hurricane center cool, dry air sinking toward the ocean surface
A hurricane consists of the central eye, which can be 5–120 miles (8–200 km) across, the eyewall (a column of thick clouds, rain, and upward-spiraling winds), and the rainbands.
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STORM SURGE
Hurricane Frances hits Juno Beach, Florida, in September 2004. Classed as a Category 2 hurricane when it hit land, Frances caused a storm surge 6 ft (2 m) high, which ripped across highways and flooded homes and business premises.
HURRICANE CATEGORIES A classification system called the Saffir–Simpson Scale divides hurricanes into five categories. It is used to estimate the damage and flooding to be expected along a coast impacted by the hurricane. Wind speed is the determining factor in the scale. CATEGORY
WIND SPEED
Tropical Storm
39–73 mph (63–118 kph)
less than 3 ft (1 m)
Category 1 hurricane
74–95 mph (119–153 kph)
3–5 ft (1–1.5 m)
Category 2 hurricane
96–110 mph (154–177 kph)
6–8 ft (2–2.4 m)
Category 3 hurricane Category 4 hurricane Category 5 hurricane
111–129 mph (178–208 kph) 130–156 mph (209–251 kph) over 156 mph (251 kph)
9–12 ft (2.7–3.7 m) 13–18 ft (4–5.5 m) over 19 ft (5.8 m)
DISCOVERY
STORM CHASERS
LOCKHEED WP-3D ORION
This turboprop aircraft, equipped with a sophisticated array of instruments, is one of those used in hurricane study.
As it moves across the ocean, the low-pressure eye of a tropical cyclone sucks seawater up into a mound, which can be up to 12 ft (3.5 m) above sea level for a Category 2 hurricane or 25 ft (7.5 m) for a Category 5. When the cyclone hits land, the water in this mound surges over the coast in what is known as a storm surge. The surge may flood homes, wash boats inland, destroy roads and bridges, and seriously erode a section of coastline up to 95 miles (150 km) wide. These effects compound the devastation caused by high winds, which can topple unstable buildings, uproot trees, damage coastal mangroves, and bring down power lines. Human deaths are not uncommon, so coastal areas threatened by a severe cyclone are normally evacuated in advance. Offshore, the water movements associated with a storm surge can devastate coral reefs. In the Caribbean, branching corals that live near the surface, such as elkhorn WATERSPOUT corals, are particularly Waterspouts are vulnerable. Healthy tornados (narrow, whirling masses of reefs can recover from air) over the sea. such damage, although They are quite it can take 10–50 years commonly spawned depending on the around the edges of tropical cyclones. extent of injury.
CORAL DAMAGE
This colony of elkhorn coral was smashed by Hurricane Gilbert on Mexico’s Caribbean coast in 1988.
INTRODUCTION
The United States’ National Oceanic and Atmospheric Administration (NOAA) monitors Atlantic hurricanes using specially equipped aircraft. They fly into hurricanes to drop instrument packages, which radio back data.
HEIGHT OF SURGE
Coastal Effects
REDUCED TO TATTERS
Survivors stand among ruined houses in Tacloban, on Leyte island in the Philippines, after its destruction by Typhoon Haiyan and the accompanying storm surge.
73
Typhoon Haiyan GENESIS
PATH OF DESTRUCTION
TRAIL OF DEVASTATION
The map below shows Haiyan’s path between November 5, when it reached the equivalent of a category 4 hurricane, and November 11, when it was downgraded to a tropical depression as it moved into China. Its most dangerous phase was reached on November 8, when it crossed the Philippines as a super-typhoon, equivalent to a Category 5 hurricane. These satellite images show Haiyan’s appearance as it centered over the Philippines (top) on Friday November 8, and as it moved into southern China (bottom) on Monday, November 11.
INFRASTRUCTURE DAMAGE The high level of destruction in urban areas, which included downed power lines and radio masts, blocked roads and interfered with relief efforts.
FLATTENED PALMS The typhoon’s effect on rural areas was equally terrible. This aerial view shows a destroyed coconut plantation and village on Leyte island.
FOOD DROPS The disaster left an estimated 1.9 million Filipinos in need of food, water, and shelter. Here, a Philippine Air Force crew drops sacks containing food supplies.
PACIFIC OCEAN
Nov 6–8 Category 5
Nov 9–10 Category 2
VIETNAM Nov 8–9 Category 4
PA L AU Nov 5 Category 4
RELIEF OPERATION
P H IL P P INE S
LAOS
MEDICAL AID Thousands needed medical assistance and urgent measures to combat spread of infectious diseases. Here, a child receives a measles vaccine.
INTRODUCTION
Nov 11 Tropical depression Nov 10–11 Tropical storm Nov 10 Nov 9 Category 1 Category 3
EVACUATION Warnings had been issued across a wide region to evacuate or seek safe refuge. Here, a Filipino child is being taken to board a military plane as part of an evacuation program.
DESTROYED COMMUNITY The destruction caused to parts of the central Philippines that Haiyan passed across was almost total. This aerial view shows the town of Guiuan in eastern Samar province.
Track of the Typhoon
CHINA
EYE OF THE STORM Haiyan originated around November 2 from an area of low pressure in the western Pacific. Over the next 3–4 days, it grew into a super-typhoon and developed a distinct “eye” at its center.
STORM SURGE By November 7, Haiyan was producing sustained winds of up to 168 mph (270 kph). The next day, it made landfall in the Philippines, where a storm surge pounded coastal areas.
LANDFALL IN PHILIPPINES
Typhoon Haiyan, known in the Philippines as Typhoon Yolanda, was an exceedingly powerful tropical cyclone that devastated the Philippines and some other parts of Southeast Asia between November 4 and November 11, 2013. It is the deadliest typhoon ever recorded in the Philippines, killing more than 6,000 people in that country alone. Haiyan is also the most powerful storm ever to hit land, and the fourth most intense tropical cyclone ever recorded in terms of highest sustained wind speeds. Most of the catastrophic destruction occurred in a central group of islands within the Philippines called the Visayas. Although wind speeds were extreme, the major cause of damage and loss of life was a storm surge, particularly hitting the eastern coasts of the islands of Samar and Leyte. The city of Tacloban, with a population of over 220,000, was almost completely destroyed. On November 10 and 11, Haiyan passed over Vietnam and southern China, by which time it had weakened to a tropical storm and then to a depression. Nevertheless, there was extensive flooding in some areas, thousands of homes were destroyed, and around 50 people were killed. Some climate scientists believe that global warming will increase the frequency of high-intensity tropical cyclones like Typhoon Haiyan. If this is the case, governments in affected countries will need to make provisions for dealing with similar-scale catastrophes more often in the future.
WAVES AND TIDES are two important
physical phenomena that affect every area of the oceans but tend to be most noticeable, and have their main effects, on or near coasts. Ocean waves are mostly wind-generated and vary from tiny coastal ripples, to the regular, rolling swell of the open ocean, to monster breakers on worldfamous surfing beaches. All waves transmit energy—when the waves reach land, this energy may be dissipated destructively, eroding coastlines, or constructively, building up features such as beaches. Tides are caused mainly by interactions between the Moon and Earth. As well as regular rises and falls in sea level, they can cause strong currents around coasts and, in some places, even more dramatic phenomena such as whirlpools and eddies.
WAVES A N D T ID E S LAPPING WAVES
Waves lapping on the seashore meet a rocky stream at low tide near Kipahulu on the southeast coast of the Hawaiian island of Maui, in the Pacific.
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WAVES AND TIDES
Ocean Waves
Wave Generation
WAVES ARE DISTURBANCES
in the ocean that transmit energy from one place to another. The most familiar types of waves—the ones that cause boats to bob up and down on the open sea and dissipate as breakers on beaches—are generated by wind on the ocean surface. Other wave types include tsunamis, which are often caused by underwater earthquakes (see p.49), and internal waves, which travel underwater between water masses. Tides (see p.78) are also a type of wave.
Wave Properties
CAPILLARY WAVES (RIPPLES)
A group of waves consists of several crests separated by troughs. The height of the waves is called the amplitude, the distance between successive wave crests is known as the wavelength, and the time between successive wave crests is the period. Waves are classified into types based on their periods. They range from ripples, which have periods of less than 0.5 seconds, up to tsunamis and tides, whose periods are measured in minutes and hours (their wavelengths range from hundreds to thousands of miles). In between these extremes are chop and swell—the most familiar types of surface wave. Ocean waves behave like light rays: they are reflected or refracted by obstacles they encounter, such as islands. When different wave groups meet, they interfere—adding to, or canceling, each other. wavelength still-water level
direction of wave motion
wave height (amplitude)
disordered sea surface in fetch area
wind direction
ripples turn to chop
outside the fetch, waves become sorted by speed and wavelength
CHOPPY SEA
In a choppy sea, the waves are 4–20 in (10–50 cm) high and have a wavelength of 10–40 ft (3–12 m).
Within the wave-generation area, the sea surface is usually quite confused—the result of groups of waves of different size and wavelength interfering with each other. Outside this area, the waves become sorted by speed to produce a more regular pattern, called a swell.
direction of wave advance fetch (area over which wind blows)
FULLY DEVELOPED ROUGH SEA
Wind speeds over 40 mph (60 kph) can generate very rough seas with waves more than 10 ft (3 m) high.
path of individual water particle
PARTICLE MOVEMENT
INTRODUCTION
As waves pass over the surface, the particles of water do not move forward with the waves. Instead, they gyrate in little circles or loops. Underwater, the particles move in ever-smaller loops. At a depth below about half the distance between crests, they are quite still.
These tiny waves are just a few millimetres high and have a wavelength of under 1½ in (4 cm).
BUILDING WAVES
crest
trough
Wind energy is imparted to the sea surface through friction and pressure, causing waves. As the wind gains strength, the surface develops gradually from flat and smooth through growing levels of roughness. First, ripples form, then larger waves, called chop. The waves continue to build, their maximum size depending on three factors: wind speed, wind duration, and the area over which the wind is blowing, called the fetch. When waves are as large as they can get under the current conditions of wind speed and size of fetch, the sea surface is said to be “fully developed.” The overall state of a sea surface can be summarized by the significant wave height—defined as the average height of the highest one-third of the waves. For example, in a fully developed sea produced by winds of about 25 mph (40 kph), the significant wave height is typically about 8 ft (2.5 m).
Wave Propagation ROGUE WAVES
Interference between two or more large waves occasionally causes a giant or “rogue” wave. This one, recorded in the Atlantic Ocean in 1986, had an estimated height of 56 ft (17 m). It broke over the ship pictured, bending its foremast back by 20˚.
In the fetch, many different groups of waves of varying wavelength are generated and interfere. As they disperse away from the fetch, the waves become more regularly sized and spaced. This is because the speed of a wave in open water is closely related to its wavelength. The different groups of waves move at different speeds and so are naturally sorted by wavelength: the largest, fastest-moving waves at the fore, the smaller, slower-moving ones behind. This produces a regular wave pattern, or swell. Occasionally, groups of waves from separate storms interfere to produce unusually large “rogue” waves. As they propagate across the open ocean, wind-generated waves maintain a constant speed, which is unaffected by depth until they reach shallow water. Only with waves of extremely long wavelength—tsunamis—is the speed of propagation affected by water depth. SWELL
A swell is a series of large, evenly spaced waves, often observed hundreds of miles away from the storm that spawned them. Wavelengths range from tens to hundreds of feet.
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PLUNGING BREAKER
“Barrel” or “tube-forming” breakers like this occur when the waves reaching shore have large amounts of energy. The seabed must be firm and quite steep.
direction of wave motion
water motion occurs offshore to depth of half the wavelength
wave shortens in length and decreases in speed but increases in height
wave reaches critical ratio of height to length and begins to break
water motion caused by the wave begins to interact with the sea bed and slow down
SHOALING AND BREAKING
Shoaling occurs as waves enter shallow water. The wave length and speed both decrease, but the wave gains height. When the crest gets too steep, it curls and breaks.
HUMAN IMPACT
RIDING THE WAVES
water carried up shore in swash zone
Arrival on Shore As waves approach a shore, the motion they generate at depth begins to interact with the sea floor. This slows the waves down and causes the crests in a series of waves to bunch up—an effect called shoaling. The period of the waves does not change, but they gain height as the energy each contains is compressed into a shorter horizontal distance, and eventually break. There are two main types of breaker. Spilling breakers occur on flatter shores: their crests break and cascade down the front as they draw near the shore, dissipating energy gradually. In a plunging breaker, which occurs on steeper shores, the crest curls and falls over the front of the advancing wave, and the whole wave then collapses at once. Waves can also refract as they reach a coastline. This concentrates wave energy onto headlands (see p.93) and shapes some types of beach (see p.106). WAVE REFRACTION
When waves enter a bay enclosed by headlands, they are refracted (bent) as different parts of the wave-front encounter shallow water and slow down.
INTRODUCTION
When a swell reaches a suitably shaped beach, it can produce excellent surfing conditions. Small spilling breakers are ideal for novice surfers, while experts seek out large plunging breakers that form a “tube” they can ride along. For tube-riding, the break of the wave must progress smoothly either to the right or left. Here, a surfer rides a rightbreaking wave in Hawaii— it is breaking from left to right behind the surfer.
wave finally breaks
WAVES AND TIDES
Tides
Tidal Patterns
TIDES ARE REGULAR RISES AND FALLS
in sea level, accompanied by horizontal flows of water, that are caused by gravitational interactions between the Moon, Sun, and Earth. They occur all over the world’s oceans but are most noticeable near coasts. The basic daily pattern of high and low tides is caused by the Moon’s influence on the Earth.Variations in the range between high and low tides over a monthly cycle are caused by the combined influence of the Sun and Moon.
High and Low Tides
If no continents existed and the Moon orbited in the Earth’s equatorial plane, the sweeping of the tidal bulges over the oceans would produce two equal daily rises and falls in sea level (a semidiurnal tide) everywhere on Earth. In practice, landmasses interfere with the movement of the tidal bulges, and the Moon’s orbit tilts to the equatorial plane. Consequently, many parts of the world experience tides that differ from the semidiurnal pattern. A few have just one high and one low tide a day (called diurnal tides), and many experience high and low tides of unequal size (known as mixed semidiurnal tides). In addition, the tidal range, or difference in sea level between high and low water, varies considerably across the globe.
0
6
12
18
24
18
24
3m/10ft 2m/6ft 1m/3ft 0m/0ft DIURNAL TIDE 0
6
12
3m/10ft
mixed semidiurnal small tidal range
This map shows the general pattern of tides (diurnal, semidiurnal, or mixed) and size of tidal range (average difference between high and low water) around the world.
1m/3ft 0m/0ft MIXED SEMIDIURNAL TIDE
medium tidal range large tidal range
L
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P A C I F I C O C E A N
OCEAN
SOUTHERN OCEAN
OCEAN
INTRODUCTION
SEMIDIURNAL TIDE
IC
Compared to permanently submerged plants and animals, organisms living in the intertidal zone have to cope with many extra stresses. They need to adapt, for instance, to the problem of becoming dried out (desiccated) when the tide is out. They may also have to endure extreme cold on frosty winter nights and even predation by land animals. Mussels, for example, often have to wait for hours between high tides to feed. At low tide, their shells close tightly to prevent desiccation and to protect against predators.
0m/0ft
T
INTERTIDAL LIFE
1m/3ft
T
bulges due to combined forces
The two ocean bulges caused by the gravitational interaction between the Earth and Moon are shown (much exaggerated) here.
2m/6ft
2m/6ft
diurnal
24
3m/10ft
GLOBAL PATTERNS
KEY
INDIAN
DAILY TIDES
Time (hrs) 12 18
6
A
Although the Moon is usually thought of as orbiting the Earth, in fact both bodies orbit around a common centre of mass – a point located inside the Earth. As the Earth and Moon move around this point, two forces are created at the Earth’s surface: a gravitational pull towards the Moon, and an inertial or centrifugal force directed away from the Moon. These forces combine to produce Earth gravitational pull of Moon creates tidal two bulges in the Earth’s oceans: bulge one towards the Moon, and the other away from it. As the Earth spins on its axis, these bulges sweep Moon over the planet’s surface, producing inertial force creates second high and low tides. The cycle tidal bulge repeats every 24 hours 50 minutes (one lunar day) rather than every 24 hours (one solar day), because centre of mass of during each cycle, the Moon Earth–Moon moves round a little in its orbit. system
Earth’s spin causes bulges to sweep over surface
0
Tide heights
78
TIDES
79
Sun SPRING TIDES
new Moon
high tide low tide
low tide
high tide
full Moon
Sun NEAP TIDES
first quarter Moon low tide
high tide
Tidal Currents
In addition to the daily cycle of high and low tides, there is a second, monthly, cycle. In this case, the Sun and Moon combine to drive the cycle. As with the Moon, the interaction between the Earth and Sun causes bulges in the Earth’s oceans, though these are smaller than those caused by the Moon. Twice a month, at the times of new and full Moon, the Sun, Moon, and Earth are aligned, and the two sets of tidal bulges reinforce each other. The result is spring tides – high tides that are exceptionally high, and low tides that are exceptionally low. By contrast, at the times of first and last quarter Moon, the effects of the Sun and Moon partly cancel out, bringing tides with a smaller range, called neaps.
The vertical variation in sea level that occurs locally with tides can happen only through horizontal flows of water, called tidal currents. Over each daily tidal cycle, the currents typically (but not invariably) run fastest about half-way between high and low tide at that location – at intermediate times they slow (“slack water”) and then reverse direction. The shape of a coast can have a crucial influence on current strength. Bottlenecks to water flow, such as narrow channels and promontories, are often associated with very powerful currents, called tidal races, that develop twice or four times a day. Where the flowing water meets underwater obstructions, phenomena such as whirlpools or vortices (spiralling, funnel-shaped disturbances), eddies (larger, flatter, circular currents), and standing waves may develop. Other tide-related phenomena include tide rips – turbulence caused by converging currents – and overfalls, defined as a tidal Wellington current flowing opposite to the wind direction.
high tide
low tide
FIRST QUARTER
ALTERNATING SPRINGS AND NEAPS
last quarter Moon
Twice a month (top), the alignment of the Sun, Moon, and Earth creates spring tides. At other times (left), when the Sun and Moon lie at right angles, it creates neap tides. The alternation between springs and neaps can be seen in the 28-day tidal graph shown below. FULL MOON
LAST QUARTER
TIME OF LOW WATER, WELLINGTON
COOK STRAIT CURRENTS
NEW MOON
TIDE HEIGHT
NEW MOON
Monthly Cycle
SPRING
NEAP
SPRING
NEAP
These maps show the pattern of strong tidal currents in the Cook Strait, between the North and South islands of New Zealand, which occur twice a day, just over six hours apart. Water must funnel through a narrow channel in the Strait.
Wellington
TIME OF HIGH WATER, WELLINGTON
SPRING
Due to tides, large swathes of coast around the world are alternately covered and uncovered by the sea. This intertidal sandflat in Northumberland, England, has a tidal range averaging about 4m (13ft).
INTRODUCTION
LOW TIDE AT BAMBURGH BEACH
80 HUMAN IMPACT
SURVIVING THE OLD SOW
MINI-VORTEX
This mini-whirlpool, about 20 ft (6 m) wide and 16 in (50 cm) deep, would be called a “piglet” by experienced Old Sow watchers. Sometimes, several of these small vortices occur, rather than a single large whirlpool. ATLANTIC OCEAN NORTHWEST
The Old Sow Whirlpool FEATURES
Tidal race, small whirlpools, occasional large whirlpool TIMING
Four times daily LOCATION Off the southern tip of Deer Island, New Brunswick, Canada
Situated at the southern end of Passamaquoddy Bay on the US– Canada border, the Old Sow is one of the largest whirlpools in the world,
ATLANTIC OCEAN NORTHEAST
Lofoten Maelstrom FEATURES
Tidal race and large, weak eddy TIMING
Four times daily LOCATION Between Lofoten Point and Mosken in the Lofoten Islands, off northwest Norway
INTRODUCTION
Also known as the Moskenstraumen, the Lofoten Maelstrom is a complex pattern of sea-surface disturbances caused by tidal flows of water over
TIDAL DISTURBANCE
For centuries, the Lofoten Maelstrom had a reputation as one of the world’s most powerful tidal phenomena.
and by far the largest in the Americas. Passamaquoddy Bay is at the lower end of the Bay of Fundy, which is famous for its strong tides. The Old Sow, when it appears, is located at a spot where various tidal streams flowing through the channels
between different islands converge during the ebb tide or diverge during the flood tide. As they flow, these currents encounter underwater obstructions, such as ledges and small
COASTAL SETTING
The Old Sow develops between Deer Island (top) and Moose Island in Maine, USA (foreground).
a broad, submerged ledge of rock between two of the Lofoten Islands. These flows result from large sea-level differences that develop four times a day between the Norwegian Sea and the Vestfjord on the eastern side of the Lofoten Islands. The word “maelstrom” originates with the tidal phenomena in this area, and is derived from the Nordic word male, meaning “to grind.” In Norse mythology, the Maelstrom was the result of a large salt-grinding millstone on the floor of the Norwegian Sea, which sucked water into its central hole as it turned. First described by the Greek explorer Pytheas in the 3rd century bc,
the Lofoten Maelstrom is marked on many historical charts as an enormous and fearsome whirlpool. In 1997, a detailed study of tidal currents in the vicinity of the island of Mosken found that the reality is somewhat different. Although some strong tidal currents were measured, no obvious large whirlpool, with a vortex, was detected. Instead, the researchers found a weak eddy, about 4 miles (6 km ) in diameter, to the north of Mosken. This eddy develops twice a day during the flood tide, when it moves in a clockwise direction, and twice on the ebb tide, when it moves slightly farther north and goes counterclockwise.
The Old Sow has caused about a dozen fatalities from drowning over the past 200 years. Most of these involved mariners who strayed too close to the whirlpool in small rowboats or sailboats. In recent times, a few people in powerboats have had anxious experiences when their engines have stalled. Experienced mariners advise that if caught in a whirlpool, the priority is to keep the boat on an even keel and avoid getting swamped. Most objects floating in a stable position will eventually spin clear. seamounts, so as they reach their maximum speed of up to 17 mph (28 km/h), the whole sea surface in this area becomes rough and disordered. Typical disturbances include standing waves, troughs (long depressions in the surface), and “boils” (smooth circular areas where water spouts up from deep below). Occasionally and unpredictably, the Old Sow itself appears, forming a vortex that can be 100 ft (30 m) wide and 10 ft (3 m) deep. More often, one or several smaller vortices, known locally as piglets, appear. As with all tidal disturbances, these phenomena are more powerful during a spring tide, which occurs a day or two after a full or new moon.
PEOPLE
JULES VERNE The French novelist Jules Verne (1828-1905) made reference to the Lofoten Maelstrom in his tale of undersea exploration, Twenty Thousand Leagues Under the Sea. At the end of the novel, Captain Nemo and his submarine, Nautilus, are sucked down into the whirlpool, “whose power of attraction extended to a distance of twelve miles,” suffering an unknown fate.
81 ATLANTIC OCEAN NORTHEAST
Saltstraumen FEATURES
Tidal race and small whirlpools TIMING
Four times daily Between Saltenfjord and Skjerstadfjord, northwest coast of Norway
LOCATION
The Saltstraumen tidal race occurs on the northwest coast of Norway and is generally acknowledged to be the strongest and most extreme tidal current in the world. It forms at a bottleneck between the Saltenfjord, an inlet from the Norwegian Sea, and the neighboring Skjerstadfjord: its driving force is a difference in sea level of up to 10 ft (3 m) that develops four times a day between the two bodies of water. The channel at the center of the bottleneck—Saltstraumen itself—is a 2-mile- (3-km-) long strait between two headlands, with a width of just 500 ft (150 m) and a depth that varies from 65 to 330 ft (20 to 100 m). Twice
a day, some 105 billion gallons (400 billion liters) of water roar through this strait on the flood tide, reaching maximum speeds of up to 25 mph (40 km/h), as tidal forces act to fill the 30-mile(50-km-) long Skjerstadfjord. Twice a day, the waters flow out again through the same channel. The flows of water, and associated whirlpools, are equally strong during the ebb as the flood tide. Despite Saltstraumen’s ferocity, the channel is regularly used by shipping. For short periods every day, the tidal flows slow almost to a halt, allowing large vessels to pass safely into and out of Skjerstadfjord. Smaller vessels do remain at risk from residual underwater currents during these periods of “slack water,” but many experienced pilots still venture out. Saltstraumen offers both interesting opportunities for divers and excellent angling (see panel, below). Incoming tides carry large amounts of plankton through the channel, and fish of various sizes follow.
DANGEROUS WATERS
When the tidal race flows, the spinoff vortices, which can be 33 ft (10 m) across, are capable of pulling objects down to the rocky bottom of the channel.
DISCOVERY
LIFE BENEATH THE WHIRLPOOLS It is possible to dive into and explore the Saltstraumen, although this can safely be attempted only when the tidal streams are at a minimum. Divers have discovered rich and colorful marine life at the bottom of the channel, dominated by long strands of kelp and a variety of invertebrates, as well as fish such as lumpsuckers, coley, and wolf-fish. TEEMING WITH LIFE
Invertebrate life at the bottom of the channel includes colorful sponges and anemones.
ATLANTIC OCEAN NORTHEAST
Corryvreckan Whirlpool FEATURES
Tidal race, standing waves, and whirlpools TIMING
Twice daily Between the islands of Jura and Scarba, west coast of Scotland, UK
LOCATION
SPIN-OFF VORTEX
In the whirlpool area, massive upthrusts of water occur in pulses, producing vortices that spin away with the tidal flow.
DISTURBED SEA
An area of disturbance begins to develop in the channel north of Jura, seen here with the island of Scarba lying behind it.
ATLANTIC OCEAN NORTHEAST
Slough-na-more Tidal Race FEATURES
Tidal race with eddies and standing waves TIMING
Four times daily Between Rathlin Island and Ballycastle Bay, County Antrim, Northern Ireland, UK
LOCATION
The Slough-na-more Tidal Race results from strong tidal flows of billions of gallons of seawater between the Atlantic Ocean and the Irish Sea, via a narrow channel. During spring tides, the tidal stream can attain a speed of 8 mph (13 km/h). Where it passes Rathlin Island, a complex of fastmoving currents, eddies, and standing waves is created. In contrast, the same sea area is usually calm during other phases of the tidal cycle. In 1915, the strength of the Slough-na-more Tidal Race forced the Irish steam coaster SS Glentow aground on the Irish coast, and the ship later broke up.
INTRODUCTION
The most famous tidal phenomenon in the British Isles can be found in the Gulf of Corryvreckan. Twice a day on the flood tide, strong Atlantic currents and unusual underwater topography conspire to produce an intense tidal race. As the tide enters the narrow bottleneck at Corryvreckan, currents of up to 14 mph (22 km/h) develop. Underwater, these currents encounter a variety of irregular features on the
seabed, including a conical obstruction known as the Pinnacle, which rises to within 95 ft (30 m) of the surface. The steep east face of this obstruction forces a plume of water to the surface, producing whirlpools and standing waves up to 13 ft (4 m) high, and the roar of the rushing water can be heard up to 3 miles (5 km) away. Over the years, the Corryvreckan has caused numerous emergencies – the author George Orwell nearly drowned there in 1947.
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TIDES AND WAVES ATLANTIC OCEAN NORTHEAST
Needles Overfalls
ATLANTIC OCEAN EAST
Garofalo Whirlpool
FEATURES
FEATURES
Tidal race and overfalls TIMING
Tidal race, small whirlpools, and overfalls
Four times daily
TIMING
PACIFIC OCEAN NORTHEAST
Yellow Bluff Tide Rip FEATURES
Tide rip, standing waves, and eddies
Four times daily
TIMING
Twice daily LOCATION Needles Channel, northwestern coast of the Isle of Wight, England, UK
Strait of Messina, between the northeast coast of Sicily and Calabria in mainland Italy
LOCATION
LOCATION
The Needles Channel is a 5-mile(7-km-) long stretch of water between a line of chalk sea stacks on one side (the Needles) and an underwater reef on the other. This stretch of water is affected by short, breaking waves (overfalls) at the time of the maximum ebb or flood tide. If the wind is blowing in the opposite direction of the tidal stream, these overfalls are greatly exacerbated, producing an extremely rough sea.
PACIFIC OCEAN NORTHEAST
Skookumchuck Narrows Tidal Race Tidal race, small whirlpools, and standing wave on flood tide
FEATURES
TIMING Four times daily; flood tide twice daily LOCATION
Skookumchuck Narrows, British Columbia,
Canada
San Francisco Bay, California, US
The Strait of Messina separates the “toe” of Italy from the Mediterranean island of Sicily. It varies in width from 2 to 10 miles (3 to 16 km) and is the site of numerous complex currents and small whirlpools that vary over the tidal cycle and hamper navigation through the Strait. In Italy, the small whirlpools that form are called garofali, but in the English-speaking world, the whole system of tidal disturbances is known as the Garofalo Whirlpool.
A tide rip is a stretch of rough, turbulent water caused by a tidal current converging with, or flowing across, another current. Thus it differs from a tidal race, which occurs where a tidal stream of water accelerates through a narrow opening in a coast. An example of a tide rip occurs at a place called Yellow Bluff in San Francisco Bay, not far from the bay’s entrance, the famous Golden Gate.
One of the world’s most famous tidal races occurs at the Skookumchuck Narrows on British Columbia’s Sunshine Coast, not far from Vancouver (Skookum is a native American word for “strong” and chuck means “water”). Four times a day, there is a strong tidal rush of water through this 1,000-ft- (300-m-) wide channel, which connects two inlets into the coast—the Sechelt and Jervis inlets. A 10-ft (3-m) difference in sea level between low and high tide causes some 167 billion gallons (760 billion
liters) of seawater to rush through the gap, creating turbulence and some small whirlpools. On the flood tide, when water is flowing into the Sechelt Inlet (but not the ebb tide, when it flows out), the tidal stream across an outcrop of bedrock in the channel creates a large standing wave—a mound of breaking water that remains stationary at a particular spot on the surface. At its peak, the flow rate is about 4.75 million gallons (18 million liters) per second, and current velocities can reach 20 mph (32 km/h).
Four times a day, strong movements of water occur through the Golden Gate—twice flowing into the bay on the flood tide and twice flowing out on the ebb tide. These currents can reach a speed of up to 5 mph (8 km/h) during spring tides. Inside the bay, the pattern of currents becomes more complex, as they either split (during the flood tide) or converge (during the ebb tide) from different parts of the bay. The currents are also modified by the varying depth of the water around the shoreline, by the shoreline’s shape, and by subsurface obstructions. At Yellow Bluff, disturbances to the sea surface are most noticeable during the ebb tide, when the tidal streams are converging, and are characterized by such phenomena as extremely rough, fast-moving water, standing waves, and eddies. The spot is popular with extreme kayakers, who challenge themselves against the strong currents and surf on the standing waves.
HUMAN IMPACT
SURF-KAYAKING
INTRODUCTION
Skookumchuck Narrows is a popular destination for enthusiasts of extreme surf-kayaking. The standing wave that arises there is up to 8 ft (2.5 m) high and 23 ft (7 m) wide and is regarded as one of the world’s great whitewater kayaking locations. When surf-kayaking, the object is to stay in the wave as long as possible, which requires strength and skill.
POWERFUL RAPIDS
Here, water is flowing right to left, from the Sechelt Inlet into Jervis Inlet. Six hours later, it flows back in the opposite direction.
TIDES AND WAVES
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PACIFIC OCEAN NORTHWEST
Naruto Whirlpool FEATURES
Tidal race and whirlpools TIMING
Four times daily Naruto Strait, between the islands of Shikoku and Awaji, Japan
LOCATION
The Naruto is a spectacular system of whirlpools that develops four times a day in a narrow channel separating the island of Shikoku (one of Japan’s main islands) from Awaji Island, a much smaller island lying off Shikoku’s northeastern coast. The channel, called the Naruto Strait, is one of several that join the Pacific Ocean to the Inland Sea, which is a large body of water lying between Shikoku and Japan’s largest island, Honshu. Four times a day, billions of gallons of water move into and out of the Inland Sea through this channel, generated by tidal variations in sea level between the Inland Sea and the Pacific Ocean of up to 5 ft (1.5 m). The tidal flows can reach speeds of up to 9 mph (15 km/h) during spring tides (that is, twice a month, around the time of a full or new moon). They create vortices up to 65 ft (20 m) in diameter where they encounter a submarine ridge. These vortices are not stationary but tend to move with the current, persisting for 30 seconds or more before disappearing. The whirlpools can be viewed from Awaji Island, from sightseeing boats that regularly negotiate the rapids, or from a 4/5-mile- (1.3-km-) long bridge that spans the Naruto Strait. HUMAN IMPACT
ARTISTIC INSPIRATION The Naruto Whirlpool has existed since ancient times. It is mentioned many times in Japanese poetry and is possibly the only tidal phenomenon to feature in a well-known piece of art, namely Whirlpool and Waves at Naruto, Awa Province, by the 19th-century Japanese artist Utagawa Hiroshige (a fragment is shown below).
A walkway hanging beneath the Onaruto Bridge, which spans the Naruto Straits, provides an excellent view of the whirlpools below.
INTRODUCTION
TROUBLED WATERS
OCEAN ENVIRONMENTS
COASTS INCLUDE SOME of the most
beautiful, but also some of the most rapidly changing, places on Earth. There are a great many forms that they can take—from cliffs composed of anything from limestone to lava, to beaches, spits, and barrier islands, river deltas, estuaries, and tidal flats. Each coast has its own unique history of formation, brought about by processes such as land rise and fall, sea-level change, glacial and volcanic action, and marine erosion and deposition. On and around these coasts, a variety of habitat types— ranging from sandy coastal dunes to salt marshes, coastal lagoons, and, in the tropics, mangrove swamps—are shaped by the interaction of tidal flows, breaking waves, discharge of river sediment, and a variety of biological and human-induced processes.
C OA S TS A N D TH E S E A S H O R E SANDSTONE COAST
This dramatic Australian coastline consists of eroded sandstone strata beautifully shaped and sculpted by an azure sea.
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COASTS AND THE SEASHORE
Coasts and Sea-level Change A COAST IS A ZONE WHERE THE LAND MEETS THE SEA—it
extends from the shoreline inland to the first significant terrain change. Coastlines constantly alter in response to sea-level change, land-based processes, wave action, and tides. Coasts can be classified into many types, some of which are contrasting—for example, two opposite forms linked to sea-level change are drowned and emergent coasts. The sea-level change itself may either have been global in nature (caused by a change in the volume of ocean water, for example) or only local (stemming from regional uplift or sinking of land).
Global Sea-level Change The most important cause of a global change in sea level is an increase or decrease in the extent of the world’s ice sheets and glaciers. This is related to Earth’s climate. If it cools, more water becomes locked up as ice, so there is less in the oceans. If it heats up (global warming), the ice melts and increases the volume of ocean water. Another cause of global sea-level change, which is also affected by climate, is a rise or fall in ocean temperature. Warming lowers ice reduced ocean sheet the density of water, so if the upper layers of the water continental crust oceanic oceans heat up, they expand and increase the depressed by crust ice total volume of the oceans. Any changes in the rises size of the ocean basins, the ocean’s containers, also impact globally on sea levels. For example, a change in activity at mid-ocean ridges can have such an effect and may be important in driving long-term sea-level change. increased ocean water
continental crust rises due to unloading of ice
GLACIAL CYCLES
OCEAN-BASIN CHANGE
During an ice age (top) the volume of ocean water is low as water is locked up in ice sheets. When the ice melts (bottom), the oceans expand, raising sea levels globally.
A slow, global rise in sea level can occur when new crust is produced at a fastspreading mid-ocean ridge. The relatively hot, buoyant new crust swells, pushing the ocean water upward.
old, dense crust
raised sea level
UPLIFTED TERRACE
This coastal region of New Zealand has experienced a localized sea-level fall in the recent geological past, as the land was significantly raised by an earthquake. What was beach is now flat clifftop.
upper mantle slow-spreading ridge
fast-spreading ridge
continental crust
younger, less dense crust has greater volume
OCEAN ENVIRONMENTS
oceanic crust depressed
SINKING ISLANDS
These two volcanic Pacific islands, Rai’atea (top) and Bora-Bora, are subsiding. Locally, the current global rise in sea level is therefore slightly exacerbated.
Local Sea-level Change
Drowned Coasts
Local sea-level change occurs when a particular area of land rises or falls relative to the general sea level. One of the main causes is tectonic uplifting of land, which occurs in regions where oceanic crust is being forced beneath continental crust (a process often associated with earthquakes). Another cause is glacial rebound, which is a gradual rise of a specific area of land after an ice sheet that once weighed it down has melted. During the last ice age, heavy ice sheets covered much of North America and Scandinavia. Since the ice melted, these regions have risen, and they continue to do so today at rates of up to a few inches a year. In contrast, other coastal areas are slowly sinking. Often, this occurs where a heavy load of coastal sediments is pushing the underlying bedrock down. A slow subsidence is occurring, for example, on the eastern coast of the US. Many volcanic islands also start to subside soon after they form. This is due to the fact that the material from which they are created cools, compacts, and then contracts, while the sea floor under them warps downward.
A drowned (or submergent) coast can be the result of either global or local sea-level rise. There are two types—rias and fjords. In a ria coast, the sea-level rise has drowned a region of coastal river valleys, forming a series of wide estuaries, often separated by long peninsulas. In a fjord coast, the sea-level rise has drowned one or more deep, glacier-carved valleys. Both types are RIA COAST characteristically irregular and indented. Due to The coastline around a significant global rise in sea level over the past Hobart, in Tasmania, Australia, was formed 18,000 years, drowned coasts are common by a rise in sea-level worldwide. Ria coasts are particularly prevalent flooding a series of river in northwestern Europe, the eastern US, and valleys. Here, the Hobart Australasia. Large numbers of fjords are present Bridge spans one such in coastal Norway, Chile, Canada, and New Zealand. drowned valley.
89
Emergent Coasts
Past Change
Emergent coasts occur where land has uplifted faster than the sea has risen since the last ice age. The causes are either activity at the edge of a tectonic plate or glacial rebound. On emergent coasts, areas that were formerly sea floor may become exposed above the shoreline, while former beaches often end up well behind the shoreline, or even on clifftops. Sometimes, staircaselike structures called marine terraces are created by a combination of uplift and waves gradually cutting flat platforms at the bases of cliffs (wave-cut platforms). Emergent coasts are typically rocky, but sometimes they have a smooth shoreline. Examples of these coasts occur on the US Pacific Coast and in Scotland, Scandinavia, New Zealand, and Papua New Guinea.
Scientists study past sea-level changes by examining rocks and fossils near shorelines. They also analyze ocean sediments to calculate past ocean temperatures and climatic properties. Over the past 500 million years, global sea levels have fluctuated by more than 1,000 ft (300 m). About 120,000 years ago, sea level was a few meters higher than it is today, but some 20,000 years ago, it was 400 ft (120 m) below today’s level. Most of the rise since then occurred prior to 6,000 years ago. From some 3,000 years ago to the late 19th century, sea level rose at about 1/254–1/127 in (0.1–0.2 mm) per year. In the late 20th century, this increased to an average of about 1⁄15 in (1.7 mm) per year.
RAISED BEACH
FOSSIL MAMMOTH TOOTH
New York Washington D.C.
NORTH AMERICA
Miami UPLIFTED CLIFFS
These marine cliffs in Crete, Greece (right), have been uplifted by tectonic activity, eroded, and finally tilted from the horizontal, also by tectonic activity.
THEN AND NOW
ATLANTIC OCEAN
The red dotted line on this map shows where the east coast of North America was 15,000 years ago. At that time, mammoths roamed on what is now continental shelf—it is not uncommon for a mammoth tooth (above) to turn up in fishing trawls from these areas.
OCEAN ENVIRONMENTS
In this bay in the Hebridean Islands, Scotland (left), the green areas behind the beach are former beaches that have been raised by glacial rebound since the end of the last ice age.
SURROUNDED BY WATER
The Italian city of Venice currently floods up to 200 times a year and is severely threatened by future sea-level rise, although a project to build a tidal barrier is expected to be completed in 2016.
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Global Warming and Sea-level Rise EFFECTS OF GLOBAL WARMING
PERUVIAN ANDES Global warming is having a marked impact on the world’s glaciers. The majority have shrunk since 1975, as their ice has melted faster than new ice has formed. These photographs from the same viewpoint show the extent of a glacier in the Cordillera Blanca, Peru, in 1980 (left) and 2002 (right). FUNAFUTI ATOLL This atoll is part of Tuvalu, a group of small, low-lying Pacific islands whose future existence is threatened by sea-level rise.
SUBMERGING ISLANDS
complex and, until satellite-based techniques were introduced in 1992, was somewhat imprecise. During the 1980s, a consensus emerged that sea level had been rising at 1/32 –1/8 in (1–3 mm) per year since 1900, whereas the new satellite techniques indicate a current average rise of 1/8 in (3 mm) per year. Since 1900, there has also been a rise in the temperature of Earth’s atmosphere and oceans (global warming) of about 1.44˚F (0.8˚C). There are two plausible mechanisms by which the temperature rise might be linked to the sea-level rise: first, through melting of glaciers and ice sheets, which increases the amount of water in the oceans; and second, through the expansion of seawater as it warms. Since there are no other convincing explanations of what might be causing the sea-level rise, the view of most scientists is that global warming is the cause. Based on different models of the future course of global warming (which most scientists now believe is linked to human activity), it is possible to make various predictions of how sea level will change in the future. For example, the Intergovernmental Panel on Climate Change predicts that, by the end of the 21st century, there will be a further sea-level rise of 11–381/2 in (0.28–0.98 mm). This rise will displace tens of millions of people living in low-lying coastal areas and have a devastating effect on some small island nations. Continued global warming will eventually melt the Greenland Ice Sheet, raising sea levels by about 23 ft (7 m), flooding most of the world’s coastal cities.
GLACIER RETREAT
The measurement of global sea-level change is
HIGH TIDE Homes on Funafuti Atoll are already flooded by lagoon waters from time to time during exceptionally high tides.
OC EA N
TIC
ATLA N
Gulf of Mexico
Miami
flooded area
61/2 -ft (2-m) rise
Miami
Galveston New Orleans Gulf of Mexico
20-ft (6-m) rise
OC EA N
ATLA NT IC
Jacksonville Georgetown
Miami
ANIMALS IN DANGER
Gulf of Mexico
ATLA NT IC
Jacksonville Georgetown
OC EA N
31/4 -ft (1-m) rise
Galveston New Orleans
CITY UNDER WATER Dhaka, the capital of Bangladesh, and about three-quarters of the country’s land area, is less than 27 ft (8 m) above sea level. Much of the country would be flooded by melting of the Greenland Ice Sheet. A rise of 2 ft (65 cm) would cause loss of 40 percent of productive land in southern Bangladesh. About 20 million people in coastal areas are affected by salinity in drinking water now. STARVED TO DEATH Polar bears are one of the animal species most severely threatened by global warming. The bears use Arctic sea ice as their summer hunting ground, and as the extent of sea ice diminishes, so do their opportunities for hunting and feeding.
OCEAN ENVIRONMENTS
The maps below indicate the areas of the southeastern US that would be threatened by sea-level rises of 31/4 ft (1 m), 61/2 ft (2 m), and 20 ft (6 m). A 31/4-ft (1-m) rise is a little above the upper end of estimates for what can be expected this century. With this rise, parts of Florida and southern Louisiana would be inundated up to 18 miles (30 km) from the present coastline. A rise of 20 ft (6 m) — which would be exceeded if the Greenland Ice Sheet were to melt completely — would submerge a large part of Florida, while Louisiana would be flooded as much as 50 miles (80 km) in from the present Jacksonville coastline. This would appear Georgetown to be highly unlikely in this century but could happen within a few hundred years Galveston New Orleans if global warming continues.
POPULATIONS AT RISK
Sea-level Rise in Southeastern US
92
COASTS AND THE SEASHORE
Coastal Landscapes A GREAT VARIETY OF LANDSCAPES ARE FOUND
along the coastlines of the world’s oceans. Coasts are shaped by processes such as sea-level change and wave erosion, as well as by land-based processes such as weathering, erosion and deposition by rivers, glacier advance and retreat, the flow of lava from volcanoes, and tectonic faulting. Some coastal features are made by living organisms, including the reefs built by corals and the harbors, coastal defenses, and artificial islands built by humans.
FRINGING REEF
This reef-fringed coast, around the south Pacific island of Bora Bora, is a secondary coast, as it has been modified by the activities of living organisms, notably corals.
Classification of Coasts Coasts can be classified as either primary or secondary. Primary coasts have formed as a result of land-based processes, such as the deposition of sediment from rivers (forming deltas), land erosion, volcanic action, or rifting and faulting in Earth’s crust. Coasts formed as a result of recent sea-level change, which include drowned coasts and emergent coasts (see pp.88–89), are also usually considered primary, as are coastlines consisting mainly of wind-deposited sand, glacial till, or the seaward ends of glaciers. Coasts are considered secondary if they have been heavily shaped by marine erosional or depositional processes, or by the activities of organisms, such as corals, mangroves, or, indeed, people. A few coasts—for example, emergent coasts that have undergone significant marine erosion—display both primary and secondary features and so fit into an intermediate category.
VOLCANIC COAST ARTIFICIAL COAST
OCEAN ENVIRONMENTS
Singapore Harbor, in Southeast Asia, is an example of a coast that has been heavily shaped by human activity. Before human intervention, it was a mangrove-lined estuary.
SEA ARCH
This spectacular arch in southern England is known as Durdle Door. A remnant of a once much larger headland, it is a classic feature of a marine-eroded coast.
This land-eroded volcanic cone is in the Galápagos Islands. The entire coastline around these islands was formed by volcanic activity and so is a primary coast.
COASTAL LANDSCAPES
Wave-erosion Coasts
energy concentrated on headland as wave front refracts
beach
Of all the different types of coastal landscape, perhaps the most familiar are wave-eroded cliffed coasts, a type of secondary coast. Wave erosion on these coasts occurs through two main mechanisms. First, waves hurl beach material against the cliffs, which abrades the rock. Second, each wave compresses air within cracks in the rocks, and on reexpansion the air shatters the rock. Where waves encounter headlands, refraction (bending) of the wave fronts tends to focus their erosive energy onto the headlands. At these headlands, distinctive features tend to develop in a classic sequence. First, deep notches and then sea caves form at the bases of cliffs on each side of the headland. Wave action gradually deepens and widens these caves until they cut through the headland to form an arch. part of wave Next, the roof of the arch collapses to leave front opposite an isolated rock pillar called a stack, and headland slows as it encounters finally the stack is eroded down to a stump. shallower water
93
part of wave front opposite beach continues forward
erosion eventually divides headland into stacks
lobe of sediment
CONCENTRATION OF WAVE ENERGY
When a wave front reaches a shore consisting of bays and headlands, it refracts in such a way that wave energy tends to be concentrated onto the headlands.
wave front (extended crest of wave)
UNDERCUT CLIFF
SEA CAVE
SEA STACKS
Wave action has eroded a notch, and an adjoining platform, at the base of this cliff in the Caribbean.
This deep indentation and sea cave have been eroded into cliffs in the Algarve, Portugal.
The Old Harry Rocks are chalk sea stacks at a headland near Swanage in southern England. river current
salt marsh spit
headland
movement of sand along beach backwash
swash
Marine-deposition Coasts Marine depositional coasts are formed from sediment brought to a coast by rivers, eroded from headlands, or moved from offshore by waves. An important mechanism in their formation is longshore drift. When waves strike a shore obliquely, the movement of surf (swash) propels water and sediment up the shore at an angle, but backwash drags them back down at a right angle to the shore. Over time, water and sediment are moved along the shore. Where the water arrives at a lower-energy environment, the sediment settles and builds up to form various depositional features, including spits, baymouth bars, and barrier islands (long, thin islands parallel to the coast).
second most common wind and wave direction
direction of longshore drift
prevailing wind and wave direction
SPIT FORMATION
BAYMOUTH BAR
Where a spit extends most or all of the way across the mouth of a bay or estuary, the result is called a baymouth bar. Here, a bar across the mouth of an estuary in Scotland, and an older spit, have created a sheltered coastal area of sandflats and salt marshes.
CLATSOP SPIT
This aerial view shows the impressive Clatsop Spit, at the mouth of the Columbia River in Oregon. The spit extends for 2½ miles (4 km) across the river mouth and is still growing.
OCEAN ENVIRONMENTS
On this coastline, sand and water is carried past the headland by longshore drift, but the sand settles at the mouth of an estuary where the waves are opposed by the sluggish outflow from a river. There it forms a slowly growing spit—a sandy peninsula with one end attached to the land.
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COASTS AND THE SEASHORE ATLANTIC OCEAN NORTHWEST
Greenland Ice Coast TYPE
Primary coast
Extension of ice-sheet to sea level in outlet glaciers FORMATION
EXTENT About 600 miles (1,000 km) LOCATION
Parts of western and eastern coasts of
Greenland
An ice coast forms where a glacier extends to the sea, so that a wall of ice is in direct contact with the water. This is a common feature around the highly indented margins of Greenland, mainly at the landward end of long fjords. Together, these ice walls form an interrupted ice coast, and they are the source of enormous numbers of icebergs, many of which escape the fjords and eventually reach the Atlantic. The ice coast extends along only a fraction of the total Greenland coastline, which is an astonishing 27,500 miles (44,000 km) long.
ATLANTIC OCEAN NORTHWEST
Acadia Coastline TYPE
Primary coast
Glaciation, then drowning by sea-level rise FORMATION
EXTENT
41 miles (66 km)
Southeast of Bangor, Maine, northeastern US
LOCATION
OCEAN ENVIRONMENTS
ICE COAST NEAR CAPE YORK
STAIRWAY TO THE SEA
The tops of the columns form stepping stones that first lead up from the foot of the cliff to a mound and then progress downward until they dip below the sea.
The coastline of Acadia in Maine is one of the most spectacular in the northeastern US. It now forms the Acadia National Park, most of which
is found within a single large island, Mount Desert Island, and some smaller associated islands. Sea-level rise since the last ice age has separated these islands from each other and from the mainland. The mountains that make up the basis of this coastline began to form 500 million years ago from seafloor sediments. Magma (molten rock) rising up from Earth’s interior intruded into and consumed these sedimentary rocks, producing a mass of granite that was gradually eroded to form a ridge. About 2–3 million years ago, a huge ice sheet started to blanket the area, depressing the land and sculpting out a series of mountains
MOUNT DESERT ISLAND
The south-facing coast of Mount Desert Island consists of a series of fractured granitic steps that were produced by the action of glaciers some 100,000 years ago.
separated by U-shaped valleys. Since the ice sheet receded, the land has gradually rebounded upward, but global sea-level rise has caused the Atlantic to overtake the rebound at a rate of 2 in (5 cm) per century. Today, waves and tidal currents are major agents of change at Acadia, gradually eroding the cliffs and depositing rock particles mixed with shell fragments at coves around the coastline.
COASTAL LANDSCAPES ATLANTIC OCEAN NORTHWEST
HUMAN IMPACT
Hatteras Island TYPE
CAPE HATTERAS LIGHTHOUSE
Secondary coast
FORMATION Deposition of sediment by waves and currents EXTENT
Erosion and deposition often cause shorelines to migrate. In 1999, the Cape Hatteras Lighthouse was moved because the sea had begun to lap at its base, threatening its destruction.
70 miles
(112 km) LOCATION Off the coast of North Carolina, northeastern US
Hatteras Island is a classic barrier island of sandy composition. It runs parallel to the mainland and is long and narrow, with an average width of 1,500 ft (450 m), and has been shaped by complex processes of deposition effected by ocean currents and waves. It is part of a series of barrier islands called the Outer Banks and has two distinct sections, which join at a promontory called Cape Hatteras. The dangerously turbulent waters in this area have resulted in hundreds of shipwrecks over the centuries.
ATLANTIC OCEAN WEST
Les Pitons TYPE
Primary coast
Volcanic lava-dome formation followed by volcano collapse and erosion
FORMATION
EXTENT
41/2 miles (7 km)
Southwestern coast of St. Lucia, Lesser Antilles, eastern Caribbean
LOCATION
The southwestern coastline on the Caribbean island of St. Lucia is rocky, highly indented, and steeply shelving. A landmark here is Les Pitons (“The Peaks”), two steep-sided mountain spires, each more than 2,430 ft (740 m)
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HATTERAS SHORELINE
Hatteras Island is a typical barrier island, being low-lying with wave-straightened shorelines.
NEW POSITION
The lighthouse is now located about 1,500 ft (450 m) back from the shoreline.
high. These are the eroded remnants of two lava domes (large masses of lava) that formed some 250,000 years ago on the flank of a huge volcano. The volcano later collapsed, leaving behind the peaks and other volcanic features in the area. The volcanic rocks on this coast are densely vegetated, except on the very steepest parts of Les Pitons themselves. Beneath the sea are some scattered coral reefs within a series of protected marine reserves. This region was declared a World Heritage Site in 2004. TWIN PEAKS
In this view, Petit Piton is the nearer peak, while Gros Piton, which is slightly higher and much broader, is visible in the background.
ATLANTIC OCEAN NORTHEAST
Giant’s Causeway TYPE
Primary coast
Cooling of basaltic lava flow from an ancient volcanic eruption FORMATION
EXTENT 3/5
mile (1 km)
Northernmost point of County Antrim, Northern Ireland, UK
LOCATION
OCEAN ENVIRONMENTS
The Giant’s Causeway is a tightly packed cluster of some 40,000 columns of basalt (a black volcanic rock). It is located at the foot of a sea cliff that rises 300 ft (90 m) on the northern coast of Northern Ireland. Although legend says the formation was created by a giant named Finn McCool, it in fact resulted from a volcanic eruption some 60 million years ago, one of a series that brought about the opening up of the North Atlantic. The eruption spewed up vast amounts of liquid basalt lava, which cooled to form the columns. They are up to 42 ft (13 m) tall and are mainly hexagonal, although some have four, five, seven, or eight sides.
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COASTS AND THE SEASHORE ATLANTIC OCEAN NORTHEAST
Gruinard Bay TYPE
Primary coast
Ice-sheet retreat and postglacial rebound FORMATION
EXTENT
8 miles (13 km)
LOCATION West of Ullapool, northwestern Scotland, UK
Around Gruinard Bay in Scotland there is evidence of a phenomenon known as postglacial rebound, in which a landmass, once pushed down
ATLANTIC OCEAN NORTHEAST
White Cliffs of Dover TYPE
Secondary coast
FORMATION Marine erosion of a large mass of ancient chalk EXTENT
11 miles (17 km)
LOCATION Southeastern coast of England, to east and west of Dover, UK
One of England’s most famous natural landmarks, the White Cliffs of Dover run along the northwestern side of the Strait of Dover, the narrowest part of the English Channel. They are complemented on the French side of the Strait by similar cliffs at Cap Blanc
by the huge weight of ice sheets during the last glacial period, rises again. In some areas, such as Scotland and Scandinavia, this upward rebound has outstripped the sea-level rise caused by the ice sheets melting. At Gruinard Bay, rebound is indicated by its raised beaches—flat, grassy areas behind the present-day beaches. Over the last 11,000 years, this part of Scotland has been moving upward relative to sea level, up to 4 in (10 cm) per century. RAISED BEACH
The green area beyond the present-day beach, well above the line of high tide, is the remnant of an ancient beach.
Nez. The chalk from which the cliffs are composed was formed between 100 million and 70 million years ago, when a large part of what is now northwestern Europe was underwater. The shells of tiny planktonic organisms that inhabited those seas gradually accumulated on the sea floor and became compressed into a layer of chalk that was several hundred yards thick. Subsequently, as the sea level fell during successive ice ages, this mass of chalk lay above the sea, and it later formed a land bridge between present-day England and France. However, about 8,500 years ago, the buildup of a large lake in an area now occupied by the southern North Sea caused a breach
ATLANTIC OCEAN NORTHEAST
Devon Ria Coast TYPE
Primary coast
Former river valleys drowned by sea-level rise FORMATION
EXTENT
About 60 miles
(100 km) Between Plymouth and Torbay, southwestern coast of England, UK
LOCATION
Much of the south coast of the English county of Devon consists of the drowned valleys of the Dart, Avon,Yealm, and Erme rivers, and the Salcombe–Kingsbridge Estuary. The inlets, also known as rias, are separated by rugged cliffs and headlands. This beautiful coastal area was formed by the partial flooding of valleys, through which small rivers once flowed, as a result of global sea-level rise since the last ice age. The rise in sea level has been accentuated by the fact that the southern parts of the British Isles have been tipping downwards since the last glacial maximum at a rate of up to 3 in (7 cm) per century. in the land-bridge. It eroded rapidly, causing flooding of the area that now forms the English Channel. Today, the cliffs at Dover continue to be eroded at an average rate of an inch or two per year. Occasionally a large chunk detaches from the cliff edge and falls to the ground. Many marine fossils have been discovered in the cliffs, ranging from sharks’ teeth to sponges and corals.
SALCOMBE–KINGSBRIDGE ESTUARY
The highly scenic Salcombe Estuary is the largest of the five rias on the south Devon coast. Its protected waters provide ideal conditions for sailing.
ATLANTIC OCEAN NORTHEAST
Cape Creus TYPE
Primary coast
Land-eroded rocky coastline of schists and other metamorphic and igneous rocks
FORMATION
EXTENT
6 miles (10 km)
Northeast of Girona, northeastern Catalonia, Spain
LOCATION
HIGH CHALK CLIFFS
Up to 330 ft (100 m) high, these cliffs owe their remarkable appearance to the almost pure chalk of which they are composed.
OCEAN ENVIRONMENTS
AN EASTERLY POINT OF THE CAPE
Cape Creus marks the point where the mountains of the Pyrenees meet the Mediterranean Sea. It has one of the most rugged coastlines in the entire Mediterranean region, with cliffs made of extremely roughtextured rocks, interspersed by small coves. Designated as a natural park in 1998, Cape Creus also boasts a varied underwater marine life, and is rich in invertebrate animals such as sponges, anemones, fan worms, and red corals. As such, it is a popular diving location. The landscape is said to have inspired the Spanish surrealist artist Salvador Dali (1904–1989), and it features in many of his paintings, including The Persistence of Memory.
COASTAL LANDSCAPES
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ATLANTIC OCEAN NORTHEAST
Western Algarve TYPE
Secondary coast
Erosional action of waves on ancient rock strata FORMATION
84 miles
EXTENT
(135 km) Southern and southwestern coast of Portugal
LOCATION
BALANCED STACK
At Marinha Beach near Carvoeiro, wind and waves have produced distinctive rock formations, such as this eroded sea stack balanced on the shoreline.
ATLANTIC OCEAN EAST
ATLANTIC OCEAN EAST
Nile Delta
Amalfi Coast TYPE
The western Algarve coast extends from the city of Faro in southern Portugal to Cape St.Vincent, at the southwestern tip of the Iberian Peninsula, and then for a further 30 miles (50 km) to the north. This coastline, which is bathed by the warm Gulf Stream, is notable for its picturesque, honey-colored limestone cliffs, small bays and coves, sheltered beaches of fine sand, and emeraldgreen water. Many stretches of this coast show typical features of marine erosion at work, including caves at the feet of cliffs, grottoes, blowholes, arches forming through headlands, and sea stacks (isolated pillars of rock set off from headlands). Although limestone is a primary component of the landscape, other rocks, including sandstones and shales, form parts of the cliffs along scattered stretches of the coast. The strikingly beautiful scenery has made this coast a popular vacation destination.
Secondary coast
TYPE
EXTENT
FORMATION
EXTENT
Southern side of the Sorrento Peninsula, south of Naples, southern Italy
LOCATION
North of Cairo, northern Egypt
The Nile Delta is one of the world’s largest river deltas. As with all deltas, its shoreline is classed as a primary coast because it formed as a result of sediment deposition from a river, a land-based process. The flow of water that formed it and nourishes it has been reduced significantly by the Aswan Dam in Upper Egypt and by local water usage. The sand belt at the delta’s seaward side, which prevents flooding, is currently eroding, and anticipated future rises in sea level pose a severe threat to its agriculture, freshwater lagoons, wildlife, and reserves of fresh water.
PROTECTIVE BELT
CLIFFS AT SANT ELIA POSITANO
In this satellite view, the sand belt at the front of the Nile Delta is clearly visible. The protrusions through the sand belt mark the mouths of two Nile tributaries.
OCEAN ENVIRONMENTS
Stretching along the southern edge of the Sorrento Peninsula, south of Naples, the Amalfi Coast is famous for its steep cliffs punctuated by caves and grottoes, and for its picturesque coastal towns, some of which are built into the cliffs. The inclined layers of limestone rock that form the cliffs lie at the foot of the Lattari Mountains and were formed between 100 and 70 million years ago.
150 miles
(240 km)
43 miles (69 km)
LOCATION
Primary coast
Deposition of sediment at mouth of Nile River
Marine erosion of folded and inclined limestone rock strata
FORMATION
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COASTS AND THE SEASHORE ATLANTIC OCEAN SOUTHEAST
Skeleton Coast TYPE
Secondary coast
FORMATION
Wind-formed desert dunes EXTENT 500km (310 miles) LOCATION Extending northwest from the city of Swakopmund on the coast of Namibia
The Skeleton Coast is an arid coastal wilderness in southwestern Africa, where the northern part of the Namib Desert meets the South Atlantic. Some stretches of this coast are dominated by sand dunes that extend to the sea; others consist of low gravel plains. An important influence on this largely straight coastline is the Benguela Current, a surface current that flows in a northerly direction offshore, bringing cool waters from the
direction of Antarctica. Prevailing southwesterly winds blow onto the coast from the Atlantic, but as they cross the cold offshore water, any moisture in the air condenses. This leads to an almost permanent fog bank and allows strange desert plants such as Welwitschia mirabilis, a species that survives for hundreds of years, to thrive. The coast is home to a large seal colony at Cape Fria in the north and includes many salt pans.
HUMAN IMPACT
SHIPWRECKS The Skeleton Coast is aptly named. Its frequent fogs, onshore winds, and pounding surf have made it a graveyard for both ships and sailors. Behind the coast is a steep mountain escarpment, so before the days of rescue parties, the escape route for shipwrecked mariners was a long march along the coast through an arid desert. WOODEN SKELETON
This wreck of a wooden vessel is one of many ships that have foundered on this treacherous coast.
HIGH DUNES AND POUNDING SURF
The coast’s high dunes present everchanging contours as they are blown by strong southwesterly winds. Below the dunes, waves pound the beaches.
INDIAN OCEAN NORTHWEST
OCEAN ENVIRONMENTS
Red Sea Coast TYPE
Primary coast
Faulting and sinking of land FORMATION
EXTENT 1,900km (1,200 miles) LOCATION Coasts of Egypt, Sudan, Eritrea, and Saudi Arabia, from gulfs of Suez and Aqaba to Djibouti
The Red Sea was created as a result of a rifting process that has been gradually separating Africa from the Arabian Peninsula for the past 25 million years. Rifting is the splitting of a region of Earth’s crust into two parts, which then move apart, creating a new tectonic plate boundary. This process begins when an upward flow
of heat from the Earth’s interior stretches the continental crust, causing it to thin, and eventually it may fracture, or fault. Sections of crust may sink, and if either end of the rift connects to the sea, flooding will occur, creating new coasts. On both sides of the Red Sea, there is evidence of the downward movement of blocks of crust, in the form of steep escarpments (lines of mountains). The Red Sea shoreline itself shelves steeply in many parts. On the land side, the coast is sparsely vegetated because of the region’s hot, dry climate, but underwater there are many rich and spectacular coral reefs. SEA MEETS DESERT
The steep Sarawat mountain escarpment that runs the length of the coast can be seen in the distance in this view of the Red Sea coast of the Sinai Peninsula.
COASTAL LANDSCAPES INDIAN OCEAN NORTHWEST
Tigris Euphrates Delta TYPE
INDIAN OCEAN NORTHEAST
SATELLITE VIEW
The delta’s seaward edge has advanced by about 250km (150 miles) in the past 3,000 years.
Krabi Coast TYPE
Primary coast
Chemical erosion of limestone followed by drowning FORMATION
Primary coast
Sediment deposition from Tigris, Euphrates, and Karun
FORMATION
EXTENT 160km (100 miles)
150 km (95 miles)
EXTENT
LOCATION
Andaman Sea coast of southwestern
Thailand
Parts of southeastern Iraq, northeastern Kuwait, and southwestern Iran
LOCATION
The area around Krabi on the western coast of southern Thailand is notable for its fantastic-looking formations of partially dissolved limestone, known as karst. This limestone was originally formed about 260 million years ago. At that time, a shallow sea covered what is now south Asia and slowly built up deposits of shells and coral that sediments washed in from the land subsequently buried. These formed layers of limestone, which
The Tigris Euphrates delta is a broad area of marshes and alluvial plain at the northern head of the Arabian Gulf, formed from sediment deposited by three major rivers. An important wildlife haven, the delta suffered great ecological damage between the 1970s and 2003 from various drainage and damming schemes carried out for military and political purposes. Fisheries and several animal species became threatened. Some recovery from the damage has occurred since 2003.
INDIAN OCEAN SOUTHEAST
The Twelve Apostles TYPE
Secondary coast
Wave erosion of cliffs producing large sea stacks FORMATION
EXTENT
3km (2 miles)
Near Port Campbell, southwest of Melbourne, Victoria, southeastern Australia
LOCATION
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were later thrust upwards and tipped over at an angle when India began to collide with mainland Asia some 50 million years ago. Around Krabi and Phang Nga Bay to its north, chemical erosion of these limestone strata by rainwater, followed by sea-level rise, has created thousands of craggy karst hills and islands. These include a number of isolated cone- and cylinder-shaped karst towers that rise out of the sea to heights of up to 210m (700ft) and groups of towers that sit on broad masses of limestone. Many of these karst formations are elongated in a northeast-southwest direction, reflecting the axis (or strike line) around which the original layers of limestone were tipped. KOH TAPU ISLAND
Some of the karst formations along this coast have been weathered into unusual shapes, as in these examples at Koh Tapu Island in Phang Nga Bay to the north of Krabi.
One of Australia’s best-known geological landmarks is a group of large sea stacks formed through the erosion of 20-million-year-old limestone cliffs. Known as the Twelve Apostles, even though there were originally only nine of them, the stacks are up to 70m (230ft) tall. In 2005, one of the stacks collapsed, leaving just eight. Collapses such as this are quite common and are an integral part of the erosion process.
ONGOING EROSION
The effects of wave erosion can clearly be seen at the bases of the remaining Apostles.
VICTORIA HARBOUR
This view shows Victoria Harbour with Hong Kong Island on the left and Kowloon on the right. Visited by more than 200,000 ships per year, the harbour is one of the world’s busiest.
PACIFIC OCEAN WEST
Hong Kong Harbour TYPE
Secondary coast
Artificial coast built around various natural harbours and nearby islands FORMATION
40km (25 miles)
from a satellite island, Ap Lei Chau. The margins of all these harbour areas have been artificially modified by the construction of concrete piers, seawalls, jetties, and other structures. This coastline can be classified as a secondary coast because it has been modified by living organisms, in this case, humans. In the whole of the Hong Kong region, more than 100km (60 miles) of coastline have been artificially constructed or modified.
OCEAN ENVIRONMENTS
EXTENT
Southeast of Guangzhou, on the South China Sea coast of southeastern China
LOCATION
A number of natural harbours surround Hong Kong Island, which is the best-known part of the Hong Kong region of China. The largest, naturally deepest, and most sheltered of these harbours is Victoria Harbour, which has an area of over 42 square km (16 square miles) and is situated between Hong Kong Island and Kowloon Peninsula. Other smaller harbours include Aberdeen Harbour, which separates Hong Kong Island
THE SKELETON COAST
On Africa’s desolate Skeleton Coast, the huge, steep-faced dunes of the Namib Desert meet the cold waters of the southern Atlantic. On this part of the coast, the remains of an ancient shipwreck can be seen close to the shoreline. Above it, fog hangs in the air from condensation of moisture blown in by southwesterly winds.
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COASTS AND THE SEASHORE PACIFIC OCEAN WEST
Ha Long Bay TYPE
Primary coast
Chemical dissolution and drowning of limestone formations FORMATION
EXTENT
75 miles
(120 km) LOCATION On the Gulf of Tonkin, east of Hanoi, Northeastern Vietnam
PACIFIC OCEAN WEST
PACIFIC OCEAN NORTHEAST
Huon Peninsula TYPE
Ha Long Bay is a distinctive region on the coast of Vietnam, within the Gulf of Tonkin. It consists of a body of water filled with nearly 2,000 islands composed of karst (limestone partially dissolved by rainwater). This landscape, which covers an area of just over 585 square miles (1,500 square km), was created by sea-level rise and flooding of a region with a high concentration of karst towers. Several of the islands are hollow and contain
Puget Sound
Primary coast
TYPE
Uplift of fossil coral reefs as a result of tectonic plate movement FORMATION
OCEAN ENVIRONMENTS
EXTENT
Primary coast
FORMATION
Glacier-carved coastal channels and bays EXTENT
90 miles
(150 km)
50 miles (80 km)
LOCATION Eastern Papua New Guinea, north of Port Moresby
LOCATION
For hundreds of thousands of years, the Huon Peninsula has been forced upward at a rate of about 10 in (25 cm) per century by movements of Earth’s crust at a tectonic plate boundary. This activity has pushed coastal coral reefs above the shoreline to form a series of terraced reefs on land. The oldest of these are hundreds of yards back from the coast. By studying them, scientists have learned much about changes in sea level and climate over the past 250,000 years.
Puget Sound, with its numerous channels and branches, was created primarily by glaciers. About 20,000 years ago, a glacier from present-day
AERIAL VIEW OF THE PENINSULA
North and south of Seattle, Washington State, northwestern US
huge caves, and a few have been given distinctive names, such as Ga Choi (“Fighting Cocks”) Island, Man’s Head Island, and the Incense Burner, as a result of their unusual shapes. Most are uninhabited. The Bay’s shallow waters are biologically highly productive and sustain hundreds of species of fish, mollusks, crustaceans, and other invertebrates, including corals. Designated a World Heritage Site in 1998, Ha Long Bay is currently
Canada advanced over the area, covering it in thick ice. Over the next 7,000 years, glaciers advanced and retreated several times. When they finally withdrew, they left behind many deeply gouged channels and thick layers of mud, sand, and gravel deposited by meltwater. Waves and weather have since reworked the deposits, molding landforms and shoreline, and forming beaches, bluffs, spits, and other sedimentary features. SOUND SETTLEMENTS
Much of the shoreline around Puget Sound has now been settled. The town of Tacoma is seen here, with Mount Rainier in the distance.
under threat from destruction of mangroves and from pollution caused by urban development and mining activities nearby. A further problem is a high level of plastic debris jettisoned from tourist boats into the bay. TOWERING LIMESTONE
Several large karst islands, each topped with thick tropical vegetation, tower over a central area of Ha Long Bay. These islands rise up to 660 ft (200 m) above sea level.
PEOPLE
GEORGE VANCOUVER In 1792, the British sea captain George Vancouver (1757–98) became the first European to explore the area we now know as Puget Sound, as commander of the ship Discovery. He gave names to some 75 islands, mountains, and waterways in the area, and the city of Vancouver, Washington, was subsequently named after him. Vancouver named Puget Sound after a Lieutenant Puget, who took the first party ashore to explore its southern end.
COASTAL LANDSCAPES PACIFIC OCEAN NORTHEAST
Big Sur TYPE
Intermediate coast
Tectonic uplift combined with rapid wave erosion
FORMATION
EXTENT
90 miles
(145 km) Southeast of San Francisco, coast of California, US
LOCATION
The Big Sur coastline of central California, where the rugged Santa Lucia Mountains descend steeply into the Pacific Ocean, is one of the most spectacular in the US. Like much of the west coast of North America, Big Sur is an emergent shoreline, in that the coast has risen up faster than sea level
PACIFIC OCEAN SOUTHEAST
Chilean Fjordlands TYPE
Primary coast
Deep glacier-carved valleys flooded by sea-level rise FORMATION
950 miles (1,500 km)
EXTENT
LOCATION Pacific coast of southern Chile from Puerto Montt to Punta Arenas
since the end of the last ice age. This uplift has resulted from interactions at the nearby boundary between the Pacific and North American tectonic plates—this region is crisscrossed by a complex system of faults in Earth’s crust and is subjected to frequent earthquakes. At Big Sur a combination of tectonic uplift and relentless wave erosion has produced steep cliffs and partially formed marine terraces (platforms cut at the base of cliffs by waves and then lifted up). The coast is susceptible to landslides as a result of wave action, the weakening of the cliffs by faulting and fracturing, the destruction of vegetation by summer fires, and heavy winter rainfall.
PACIFIC OCEAN CENTRAL
Hawaiian Lava Coast TYPE
Primary coast
Lava flow into the sea from an active volcano FORMATION
EXTENT
14 miles (20 km)
Southeastern coast of the Big Island of Hawaii, US
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from satellite craters of the active volcano Kilauea. Lava from the Pu’u O’o crater flows some 9 miles (15 km) to the sea, where it cools and hardens to form land. This coastal landscape is a primitive scene of black beaches and dark cliffs made of rough, fractured lava. Plants begin to colonize newly formed areas of the coast within months of their formation.
LOCATION
One of the fastest ways for a coast to change shape is as a result of lava flow to the sea. On southeastern Big Island, new coast has been added intermittently since 1969 as a result of lava flows
STEAM PLUMES
As red-hot lava enters the sea, it solidifies amid huge plumes of steam. Newly forming shoreline sometimes collapses to reveal ripped-open lava tubes.
RAISED PLATFORM
In this view of part of Big Sur, a grassed-over marine terrace (the green area) is visible above the present-day cliff, with a raised ancient cliff behind it.
The Chilean fjordlands are a labyrinth of fjords, islands, inlets, straits, and twisting peninsulas, lying to the west of the snow-capped peaks of the southern Andes. The fjordlands extend for most of the length of southern Chile, as far south as Tierra del Fuego, and their total area is some 21,500 square miles (55,000 square km). Some 10,000 years ago, this region was covered in glaciers, but these have largely retreated into large ice-filled
areas within the mountains on the Chile–Argentina border called the Northern and Southern Patagonian Ice Fields. The glaciers left behind a network of long, deeply gouged valleys, which were filled by glacier meltwater and then flooded by the sea to form today’s fjords. Rainfall here is heavy, and clear skies are rare because the moisture-laden Pacific air cools and forms clouds as
it rises to cross the Andes. On the edges of the fjords, waterfalls cascade down steep granite walls, while hundreds of species of birds nest and feed around the often mist-shrouded coast and islands. Mammals that live along this coast include sea lions, elephant seals, and marine otters.
ICE-CHOKED FJORD
The calving ends of outlet glaciers, which choke the waters with icebergs, are found at the landward end of some fjords.
OCEAN ENVIRONMENTS
BATTERED BY THE SEA
The Eastern Scheldt storm-surge barrier in the Netherlands is one of the world’s largest sea defenses. Its 62 sliding steel gates are held between concrete piers.
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Coastal Defenses TYPES OF DEFENSES
Coastal defense refers to various types of engineering
In 2000, the US Federal Emergency Management Agency estimated that as many as 87,000 houses in the US are in danger of falling into the sea by the year 2060. Among them are the condemned houses, pictured below, on eroding cliffs at Governors Run in Chesapeake Bay, Maryland. In California, about 86 percent of the coast is actively eroding. Similarly, stretches of the eastern coast of England are eroding at a rate of up to 6 ft (1.8 m) a year—the highest rate in Europe. Coastal defenses can slow coastal erosion temporarily, but in the long run maintenance will become prohibitively expensive. In the end, the sea will triumph.
HARD ENGINEERING
ROCK GROYNE This consists of a pile of large rocks built out from the shore. The aim is to slow erosion by causing a local buildup of sand, but it can aggravate erosion nearby.
SOFT ENGINEERING
DUNE STABILIZATION Coastal dunes provide valuable protection against erosion if they can be stabilized and prevented from shifting. This is usually achieved by planting with grasses.
MODERN SOLUTION
Crumbling Coasts
SEA WALL A sea wall is designed to reflect wave energy. Modern walls have a curved top that prevents water from spraying over the wall in storms. A wall protects the land behind it for some years but usually increases erosion of the beach in front of it.
BEACH NOURISHMENT This involves adding large amounts of sand to a beach. Waves and tides spread the material along the coast, temporarily building up its natural defenses.
GEOTUBE A geotube is a long, cylindrical container, over 8 ft (2.3 m) in diameter, made of a durable textile or plastic and filled with a slurry of sand and water. Different types can be laid along the top of a beach, or inside a dune, or just offshore, where they reduce coastal erosion and protect beachfronts. This tube is part of the Barren Island Tidal Wetland project in Maryland.
OCEAN ENVIRONMENTS
DAMS AND STORM-SURGE BARRIERS The Netherlands has invested in an extensive series of engineering works to protect a large region of the country from future marine flooding. Known as the Deltaworks, it includes many dams and movable stormsurge barriers. The works were initiated in 1953 after a serious storm and floods killed a total of 1,835 people.
LARGE-SCALE PROTECTION
techniques aimed at protecting coasts from the sea. The threats posed by the sea fall into two main categories. First is the danger of flooding of low-lying coastal areas during severe storms. Second is the continuous gradual erosion of some coasts. There are a number of different approaches to coastal defense. To prevent flooding of low-lying regions, one solution is to build a large-scale system of dams and tidal barriers. Another is to encourage the development of natural barriers, such as salt marshes, around coasts, and to conserve existing areas of this type. A third possibility is managed retreat. Instead of trying to hold back the sea, some areas of coast are allowed to flood. The idea is that, in time, the flooded land will turn into a marsh, providing natural protection. To slow coastal erosion, various “hard” engineering techniques are commonly employed, such as the building of sea walls, breakwaters, or groynes. These methods can be effective for a while (they usually have to be rebuilt after a few decades), but are expensive and can increase erosion on neighboring areas of coast by interfering with longshore movement of sediment. “Soft” engineering techniques are more environmentally friendly. They include the temporary solution of beach nourishment (see panel, right), which has to be repeated every few years, and encouraging the development of coastal dunes.
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COASTS AND THE SEASHORE
Beaches and Dunes BEACHES ARE DEPOSITS OF SEDIMENTARY MATERIAL, ranging
in size from fine sand to rocks, that commonly occur on coasts above the low-tide line. Sources of beach material include sediment brought to a coast by rivers, or eroded from cliffs or the sea floor, or biological material such as shells. This material is continually moved on and off shore and around coasts, by waves and tides. Wind can also influence beach development and is instrumental in forming coastal dunes.
DISSIPATIVE BEACH AND DUNE
Dissipative beaches are usually made up of fine sand, and they slope at an angle of less than 5˚.
Beach Anatomy
Types of Beaches
A typical beach has several zones. The foreshore is the area between the average high- and low-tide lines. On the seaward side of the foreshore is the nearshore, while behind it is the backshore; the latter is submerged only during the very highest tides and usually includes a flat-topped accumulation of beach material called a berm. The sloping area seaward of the berm, making up most of the foreshore, is the beach face. At the top end of the beach face there are sometimes a series of crescentshaped troughs, called beach cusps. The swash zone is the part of the beach face that is alternately covered and uncovered with water as each wave arrives. Seaward of the swash zone, extending out to where the waves break, is the surf zone. The shape of a beach often alters as wave energy changes over the year.
The level of wave energy, the direction the waves arrive from, and the geological makeup of a coast all affect the type of beach that will form. Dissipative beaches are gently sloping and absorb wave energy over a broad area, while reflective beaches are steeper and shorter, and consist of coarser sediment. If a cliffed coast contains a mixture of both easily eroded and erosion-resistant rock, headlands tend to form, with crescent-shaped beaches within the bays (embayed beaches) or smaller “pocket” beaches. Both of these tend to be “swash-aligned”—the waves arrive parallel to shore and do not transport sediment along the beach. Many long, straight beaches are “driftaligned”—the waves nd arrive at an angle and inla direction of ma longshore drift sediment is moved sand along the beach by embayed beach spit longshore drift. long driftaligned beach
SWASH
The surge of water and sediment up a beach when a wave arrives is called swash. If waves reach a beach at an angle, the combined effect of swash and backwash moves material along the beach. NEARSHORE
swash-aligned beach
RANGE OF BEACHES
This imaginary coast (right) shows several beach types, ranging from a tombolo (a sand deposit between the mainland and an island) to a drift-aligned beach.
FORESHORE
pocket beach tombolo
predominant direction of wind and waves
BEACH PARTS AND ZONES
BACKSHORE
OCEAN ENVIRONMENTS
This photograph (left) shows the main zones on a beach and the locations of the berm, beach face, and beach cusps. It was taken when the sea was approaching low tide.
average low-tide line
average high-tide line surf zone
swash zone
beach face
beach cusp
berm
foredune
berm crest
107
pebbles or medium gravel 3/8 – 1/2 in (8 mm–1.5 cm) in diameter
very fine gravel 1/16 – 1/8 in (2–4 mm) in diameter
very coarse sand 1/32 – 1/16 in (1–2 mm) in diameter
Beach Composition
PEBBLES AND SHELLS
The composition of a beach at any particular location depends on the material available and on the energy of the arriving waves. Most beaches are composed of sand, gravel, or pebbles produced from rock erosion. Sand consists of grains of quartz and other minerals, such as feldspar and olivine, typically derived from igneous rocks such as granite and basalt. Other common beach-forming materials, seen particularly in the tropics, include the fragmented shells and skeletons of marine organisms. In general, higher wave energies are associated with coarser beach material, such as gravel or pebbles, rather than fine sand. Occasionally, large boulders are found on beaches—usually they have rolled down to the shore from local cliffs, but some boulders have ended up on beaches as a result of glacial transport or even backwash from tsunamis.
This high-energy beach (left) contains many large pebbles. Mollusk shells in the beach below reflect favorable offshore feeding conditions for the live mollusks.
Coastal Dunes
medium sand 1/100 – 1/50 in (0.25–0.5 mm) in diameter
fine sand 1/200 – 1/100 in (0.125–0.25 mm) in diameter coarse silt 1/850 – 1/400 in (0.03–0.06 mm) in diameter
GRAIN SIZES
The silts, sands, and gravels that make up most beaches tend to become sorted by the action of waves, with material of different sizes deposited on different parts of the beach.
MARRAM GRASS
This grass is a common colonizer of embryo dunes. It develops deep roots that allow it to tap into deep groundwater stores. The roots bind the sand together, while the grass traps more blown sand, assisting in foredune development.
Coastal dunes are formed by wind blowing sand off the dry parts of a beach. Dunes develop in the area behind the backshore, which together with the upper beach face supplies the sand. For dunes to develop, this sand has to be continually replaced on the beach by wave action. The actual movement of sand to form dunes occurs through a jumping and bouncing motion along the ground called saltation. Some coastal areas have more than one set of vegetated dunes that run parallel to the shoreline. The dunes closest to shore are called foredunes; behind them is a primary dune ridge, secondary dune ridge, and so on. These anchored, vegetated dunes are important for the protection they provide against coastal erosion. On some coasts, non-vegetated, mobile dunes occur; these move in response to the prevailing winds. They can often be anchored by planting with grasses.
This beach in the Seychelles is an example of a reflective beach because of its quite steeply shelving face. It has a distinct berm and berm crest.
OCEAN ENVIRONMENTS
REFLECTIVE BEACH
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COASTS AND THE SEASHORE ATLANTIC OCEAN WEST
ATLANTIC OCEAN SOUTHWEST
Pink Sands Beach
Copacabana Beach
Dissipative beach, protected by reefs
TYPE Embayed, dissipative beach
COMPOSITION Sand mixed with broken shells and skeletons
sand
TYPE
LENGTH
COMPOSITION
LENGTH
White
21/2 miles (4 km)
2.5 miles (4 km)
LOCATION Harbour Island, off Eleuthera, northeast of Nassau, Northern Bahamas
LOCATION
Rio de Janeiro, southeastern Brazil
One of the most famous beaches in the world, Copacabana Beach is a wide, gently curving stretch of sand between two headlands. Behind the beach lies the city of Rio de Janeiro, with green, luxuriant hills in the hinterland. The beach is crowded much of the year and is known for its beach sports and New Year’s Eve firework displays. The sea area off the beach is not always recommended for swimming, due to strong currents.
Pink Sands in the Bahamas is a gently sloping beach that faces east onto the Atlantic Ocean. It is protected from ocean currents by an outlying reef. The pale pink color of the sand comes from small, singlecelled organisms called foraminiferans, in particular, the species Homotrema rubrum, also known as the sea strawberry. The shells of these organisms are bright red or pink due to the presence of an iron salt. In parts of the Bahamas they are abundant, living on the underside of reefs. When they die, they fall to the seafloor, where they are broken by wave action and mixed with other debris, such as the white shells of snails and sea urchins, as well as mineral grains. This mixture is then finely pulverized and washed up on the shore as pink-colored sand by wave action. GENTLE SLOPE
Pink Sands is an example of a dissipative beach, on which waves break some distance from the shore, then slowly roll in, dissipating their energy across a broad surf zone.
ATLANTIC OCEAN NORTHEAST
St. Ninian’s Tombolo TYPE
Tombolo COMPOSITION
Yellow and white sand LENGTH 1/2
mile (700 m)
West coast of southern Mainland, the main island of the Shetland Isles, off Scotland, UK
OCEAN ENVIRONMENTS
LOCATION
NARROW CONNECTION
A slim, sandy tombolo extends from the Shetland island of Mainland in the foreground, to St. Ninian’s Isle.
COPACABANA LOOKING NORTH
St. Ninian’s Isle in the Shetland Isles provides a classic example of a tombolo, or ayre—a short spit of sedimentary material that connects an island to a nearby land mass or mainland. A tombolo is formed by waves curving around the back of an island so that they deposit sediment on a neighboring land mass, at the point directly opposite the island. Over time, these sediments gradually build up into a tombolo, which
typically projects at right angles to the coast and has a beach on each side. St. Ninian’s Tombolo has been in existence for at least 1,000 years, and its permanence may be due to a cobble base underlying the sand. This tombolo tends to become lower and narrower during storms as a result of destructive wave action, while during calmer weather the waves build it up again with sand carried from offshore or the nearshore. The sediment that
forms a tombolo may come from the mainland, the island, the sea floor, or a combination. Scientists have deduced that a tombolo will usually form when the island’s distance from the shore is less than two-thirds of its length parallel to the shore (the distance of St. Ninian’s Isle from the shore is only about one third of its length). In other cases, a feature called a salient may form—a sand spit that extends toward the island but does not quite reach it.
109 ATLANTIC OCEAN NORTHEAST
North Jutland Dunes TYPE
Coastal dunes
Yellow sand, marram grass
COMPOSITION
LENGTH
155 miles
(250 km) LOCATION
North and northwest coast of Jutland,
Denmark
Much of the northern coastline of Denmark’s Jutland Peninsula consists of sand dunes, which cover several thousand square miles of coast. These dunes are “active” in that they have a natural tendency to migrate along the
ATLANTIC OCEAN NORTHEAST
Porthcurno Beach TYPE
Pocket beach
COMPOSITION
Yellow-white sand, composed mainly of shell fragments LENGTH
500 ft (150 m)
Southwest of Penzance, Cornwall, southwestern England, UK
LOCATION
Porthcurno is a typical pocket beach located near Land’s End at England’s southwesternmost tip. Like all pocket beaches, it nestles between two headlands that protect the sandy cove from erosion by winter storms and strong currents. Pocket beaches are
coast, carried by wind (sand drift) and wave erosion. In some areas, attempts have been made to restrict this dune drift, to prevent sand from inundating summer houses. Some early attempts were fruitless. For example, sand fences were built into the dunes during World War II, but the dunes have since moved behind them, leaving the fences on the beach. More recently, many dune areas have been stabilized more successfully by planting with grasses and conifer trees. SHIFTING SANDS
Many sand dunes on the peninsula have marram grass growing in them, which helps constrain their movement.
common where cliffs made of different types of rock are subject to strong wave action. Rock that is especially hard and resistant to erosion forms headlands, while intervening areas of softer rock are worn down to form pocket beaches. Unlike other beaches, pocket beaches exchange little or no sand or other sediment with the adjacent shoreline, because the headlands prevent longshore drift. The sea at Porthcurno is a distinctive turquoise, possibly due to the reflective qualities of the sand, which is made mainly of shell fragments.
ATLANTIC OCEAN NORTHEAST
Chesil Beach TYPE Storm beach on tombolo COMPOSITION
CHESIL BANK
The bank is about 560 ft (170 m) wide and 50 ft (15 m) high along its entire length. The beach (left) is on its seaward side.
Gravel of
flint and chert LENGTH
LOCATION
18 miles (29 km)
West of Weymouth, Dorset, southern
England
Chesil Beach forms the seaward side of the Chesil Bank, a remarkably long, narrow bank of sedimentary material that connects the coast of Dorset in southern England to the Isle of Portland. Behind the bank is a tidal lagoon called the Fleet. Running parallel to the coast, Chesil Bank looks like a barrier island. However, because it connects the mainland to an island, it is classified as a tombolo. How Chesil Bank and its beach originally formed is debated—the most widely accepted theory is that it originally formed offshore and was then gradually moved to its current location by waves and tides. The beach is classified as a storm beach, as it is affected by strong waves because it faces southwest toward the Atlantic and the prevailing winds. Like most storm beaches, it is steep, with a gradient of up to 45 degrees, and is made of gravel.
GRANITE HEADLANDS
The headlands on either side of the beach are formed from 300- million-year-old granite. DISCOVERY
Chesil Beach’s pebbles change in size progressively from potatosized at one end to pea-sized at the other. This reveals the differences in wave energy along its length—at one end, strong waves wash smaller pebbles offshore; at the other, weaker waves wash them onshore.
OCEAN ENVIRONMENTS
GRADED PEBBLES
110 ATLANTIC OCEAN NORTHEAST
Cap Ferret
ATLANTIC OCEAN EAST
Banc d’Arguin a spit
TYPE Coastal dunes and tidal flats
Sand, grasses, forest
sand
TYPE
Coastal dunes on
COMPOSITION
LENGTH
COMPOSITION
71/2 miles (12 km)
LENGTH
Yellow
100 miles
(160 km) LOCATION
Coast of Aquitaine, southwest of Bordeaux, southwestern France
LOCATION
Between Nouakchott and Nouadhibou on the northwest coast of Mauritania, West Africa
Cap Ferret lies at the southern end of a long sand spit in western France. It separates the Arcachon Lagoon from the Atlantic Ocean and forms part of the spectacular Aquitaine coast, which at 143 miles (230 km) is the longest sandy coast in Europe. This region is characterized by a series of straight, sandy beaches backed by longitudinal sand dunes, which are the highest dune formations in Europe. They include the highest individual European sand dune, the Dune du Pilat, which rises to about 380 ft (115 m) above sea level. Behind the main dune area is a forest, originally planted in the 18th century to try to prevent the dunes from shifting. Unfortunately, this coast is undergoing serious erosion, of more than 33 ft (10 m) a year in some places, mainly because excessive urban development has degraded the vegetation cover.
The Banc d’Arguin National Park is a vast region of dunes, islands, and shallow tidal flats covering more than 4,600 square miles (12,000 square km) of the Mauritanian coast. The dunes, which consist mainly of windblown sand from the Sahara, are concentrated in the southern region of the Park. Banc d’Arguin contains a variety of plant life and is a major breeding or wintering site for many migratory birds, including flamingos, pelicans, and terns. It was declared a World Heritage Site in 1987.
SAND MOUNTAINS
INDIAN OCEAN SOUTHWEST
Jeffreys Bay TYPE Series of gently sloping, dissipative beaches COMPOSITION LENGTH
Sand
9 miles (15 km)
LOCATION West of Port Elizabeth, eastern Cape Province, South Africa
Jeffreys Bay is famous both as a highly popular surfing spot and for the large numbers of beautiful seashells that wash up on its shores. It consists of a series of wide beaches strung out along a southeast-facing stretch of the South African coastline. As a surfing destination, Jeffreys Bay is regularly ranked among the top five beaches in the world by those seeking the “perfect wave.” The most acclaimed surfing spot or wave “break”
Along the coast to the north and south of Cap Ferret, mini-mountains of pale, rippling sand are backed by an extensive vegetation cover.
SAND BANKS AT BANC D’ARGUIN
is known as Supertubes. Here, the combination of shoreline shape, bottom topography, and direction of wave propagation regularly generates waves that form huge, glassy-looking hollow tubes as they break. Other nearby wave breaks in Jeffreys Bay have been given such colorful names as Boneyards, Magna Tubes, and Kitchen Windows. Some of these waves can carry a skilled surfer several hundred yards along the beach on
a single ride. The same waves that attract surfers are also responsible for the vast numbers and wide variety of seashells that are washed up onto the beach with each tide. Conchologists have identified the shells of over 400 species of marine animals, including various gastropods, chitons, and bivalves, making the bay the most biologically diverse natural coastline in South Africa. Dolphins, whales, and seals are also seen. HUMAN IMPACT
OCEAN ENVIRONMENTS
HIDDEN DANGERS
HEADING FOR SUPERTUBES
The waves at Supertubes may be 10 ft (3 m) high and invariably break right-to-left as viewed from the shore.
Every surfing spot, including Jeffreys Bay, has dangers that would-be surfers should know about. The most important are rip currents. The enormous volume of seawater washed up on shore by the waves tends to pool at specific points on the beach and is then funneled back out to sea in swift currents. These move rapidly away from the beach, straight out through the surf zone, and can sweep unsuspecting swimmers out to sea. They can be escaped by swimming parallel to the shore. At Jeffreys Bay, there have also been rare reports of surfers being bitten by sharks, most often by the sand tiger or ragged-tooth shark.
BEACHES AND DUNES INDIAN OCEAN NORTH
INDIAN OCEAN NORTH
Anjuna Beach
Cox’s Bazar Beach
Series of embayed beaches
Dissipative coastal plain beach
TYPE
COMPOSITION
TYPE
Yellow
COMPOSITION
sand LENGTH
111
Yellow
sand 1 mile
LENGTH
(1.5 km)
75 miles
(120 km)
LOCATION
On the Arabian Sea coast, northwest of Panaji, southwestern India
LOCATION
South of Chittagong, southeastern Bangladesh
Anjuna Beach is one of the most scenic and popular of the renowned string of beaches that lie on the coast of the Indian State of Goa. The beach has an undulating shape and is broken up into several sections by rocky outcrops that jut into the sea. By reducing rip currents and crosscurrents, these outcrops help to make Anjuna one of the safest swimming beaches on the Goa coast. During the monsoon season, from June to September, much of the beach sand is stripped away and carried offshore by heavy wave action, but after the monsoons, calmer seas restore the sand deposits.
Cox’s Bazar Beach lies on a northeastern stretch of the Bay of Bengal and is the second-longest unbroken natural beach in the world —Ninety Mile Beach (see p.112) in Australia is the longest. It fronts a range of dunes and, at its southern end, a spit of land. The dunes, spit, and beach have been built up over hundreds of years through a combination of wave action and deposition of sediment from the Bay of Bengal. This is a gently sloping beach that offers safe swimming and surfing and is also popular among collectors of conch shells.
PICTURESQUE SETTING
With its calm seas and sand crescents backed by swaying palms and low, rocky hills, Anjuna has been a favored vacation destination since the 1960s.
SATELLITE VIEW OF BEACH (BOTTOM RIGHT)
INDIAN OCEAN SOUTHEAST
Shell Beach TYPE
Embayed beach
Shells of a species of cockle COMPOSITION
70 miles (110 km)
LENGTH
LOCATION
Northwest of Perth, Western Australia
SHELL BANK
Individual shells in the beach are about ½ in (1 cm) wide. Accumulations of these shells over about 4,000 years has led to the formation of a long bank along the seashore.
OCEAN ENVIRONMENTS
Shell Beach, in Western Australia’s Shark Bay, has a unique composition, consisting almost entirely of the white shells of Fragum erugatum, a species of cockle (a bivalve). The beach lies in a partially enclosed area of Shark Bay known as L’Haridon Bight. This cockle thrives here because its predators cannot cope with the high salinity of the seawater. On the foreshore of Shell Beach, the layer of shells reaches a depth of 26–30 ft (8–9 m). The shells also form the sea floor, stretching for hundreds of yards from the shoreline. On the backshore, away from the water line, many of the shells have become cemented together, in some areas leading to the formation of large, solid conglomerations. These were formerly mined to make decorative wall blocks.
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COASTS AND THE SEASHORE PACIFIC OCEAN SOUTHWEST
Ninety Mile Beach TYPE Dissipative coastal plain beach COMPOSITION
Yellow
sand LENGTH
90 miles
(145 km) LOCATION Southeast of Melbourne, Victoria, southeastern Australia
Australia’s Ninety Mile Beach, on the coast of Victoria, has a solid claim to be the world’s longest uninterrupted natural beach. The beach runs in a southwest to northeasterly direction
and fronts a series of dunes. Waves generally break too close to the beach for good surfing, and strong rip currents make the conditions hazardous for swimmers. In its northeastern part, several large lakes and shallow lagoons, known as the Gippsland Lakes, lie behind the dunes. Beneath the sea, vast plains of sand stretch in every direction and are home to a large variety of small invertebrate life, including crustaceans, worms, and burrowing mollusks. AERIAL VIEW
Facing out onto the Bass Strait, Ninety Mile Beach is subject to strong waves during the winter months.
PACIFIC OCEAN SOUTHWEST
Moeraki Beach TYPE
PACIFIC OCEAN CENTRAL
Punalu’u Beach
Embayed beach
COMPOSITION
TYPE
Pocket beach
Dark
sand and large boulders LENGTH
COMPOSITION
Black sand
2 miles (3 km)
LENGTH 1/3
Northeast of Dunedin, southeastern New Zealand
LOCATION
The beach north of Moeraki on New Zealand’s South Island is strewn with groups of large, near-spherical boulders. Scientists believe they are mineral concretions that formed over a few million years within 60-million-year-old mudstones— thick layers of sedimentary rock making up the sea floor. These mudstones were later uplifted and now form a cliff at the back of the beach. There, gradual erosion exposes and releases the boulders, which eventually roll down onto the beach.
Punalu’u Beach on Hawaii’s Big Island is a steeply shelving pocket beach. It is best known for its dramatic-looking black sand, which is composed of grains of the volcanic rock basalt. The sand has been produced by wave action on local cliffs of black basaltic lava. Punalu’u, in common with about half the land area of Hawaii, lies on the flank of Mauna Loa, the world’s most massive volcano. Lava produced by the volcano dominates the local landscape—although no lava has reached Punalu’u from Mauna Loa or the nearby active volcano Kilauea for several hundred years. The beach is a popular location for swimming and snorkeling, but underwater springs that eject cold water into the sea close to the beach can cause discomfort. Punalu’u Beach is also visited by green turtles, which come to eat seaweed off rocks at the edge of the beach and bask on the warm, heat-absorbing sand.
SUPERSIZED BOULDERS
The boulders are up to 7 ft (2.2 m) in diameter, and some weigh several tons. Some are half-buried in the sand.
PACIFIC OCEAN NORTHEAST
Columbia Bay TYPE Series of embayed beaches
OCEAN ENVIRONMENTS
COMPOSITION
Gravel
and rocks LENGTH
location by ancient glaciers, remaining there when the glaciers melted. The till has usually been reworked by wave action, with the lighter material (clay, silt, and sand) washed away and the heavier gravel and rocks sorted by size and deposited in different areas along
mile (500 m)
LOCATION
the shoreline. Such is the case in Columbia Bay, a region within Alaska’s Prince William Sound. Many of the beaches in this area have old tidal lines visible above the present ones, the result of a huge earthquake in 1964 that raised the land by 8 ft (2.4 m).
Northeast of Naalehu, Big Island, southeastern Hawaii
31 miles (50 km)
LOCATION Southwest of Valdez, southern Alaska, US
Many beaches in southern Alaska, and other beaches at high latitudes in the Northern Hemisphere, consist of gravel, small rocks, and boulders. These materials come from coarse glacial till—mixtures of clay, silt, sand, gravel, and rocks that were carried to a BEACH AND BAY
BLACK AND BLUE
The backshore area visible here, which has been colonized by plants, was foreshore prior to the 1964 earthquake.
The sand on Punalu’u is almost perfectly black, contrasting with the deep blue Pacific waters. Removal of the sand is prohibited.
BEACHES AND DUNES PACIFIC OCEAN NORTHEAST
Oregon National Dunes TYPE
Coastal dunes
Yellow sand, grasses, conifers
COMPOSITION
LENGTH
40 miles
(64 km) Southwest of Portland, Oregon, northwestern US
LOCATION
Oregon National Dunes is the largest area of coastal sand dunes in North America, extending along the coast of Oregon between the Sislaw and Coos rivers. These dunes have been created through the combined effects of coastal erosion and wind transport of sand over millions of years and extend up to 21/2 miles (4 km) inland, rising to 500 ft (150 m) above sea level. A continuum of dry and wet conditions extends through the dune area. Close to the beach are low foredunes of sand and driftwood
stabilized by marram grass. Behind these are hummocks where sand collects around vegetation. Water accumulates around the hummocks seasonally, giving them the appearance of floating islands. Behind the hummocks are further distinct regions, ranging from densely vegetated areas that become marshlike in winter to completely barren, wind-sculpted high dunes. The dunes are a popular location for various recreational activities, including riding all-terrain vehicles (ATVs) and dune buggies.
113
HUMAN IMPACT
DUNE DESTABILIZATION The use of dune buggies and ATVs, especially when raced in large numbers, may destroy the grass on the dunes, making them susceptible to wind scour. This may in turn lead to self-propagating breaches in the dune ridges. To protect the dunes, ATV usage is restricted.
SEA OF DUNES
The wind molds the sand of the dunes into wave shapes, with crests at right angles to the wind direction.
PACIFIC OCEAN NORTHEAST
Dungeness Spit TYPE
Sand spit COMPOSITION
Sand LENGTH
51/2 miles (9 km) Northwest of Seattle, Washington State, northwestern US
LOCATION
Dungeness Spit, one of the world’s longest natural sand spit, juts out from the Olympic Peninsula in Washington State. It is part of the Dungeness
National Wildlife Refuge and is as little as 100 ft (30 m) wide in places. In addition to its great length, the spit has a complex shape, the result of seasonal changes in wind and wave direction. During part of the year, these bring sandy sediments from the northwest, and at other times from the northeast. The resulting pattern of sedimentation has created a large sheltered coastal area, providing refuge for many shorebirds and waterfowl, which nest along the beach, and for Pacific harbor seals. The tidal flats nourish a variety of shellfish, and the inner bay is an important nursery habitat for several salmon species.
Tamarindo Beach TYPE
Embayed beach COMPOSITION
Yellow sand LENGTH
2 miles
(3 km) LOCATION
Northwest of San José, northwestern
Costa Rica
GROWING SPIT
The spit grows at about 15 ft (4.5 m) a year. It provides shelter for a large inner bay and an area of tidal flats.
Tamarindo Beach is a curved, gently shelving crescent of sand situated close to a mangrove-lined estuary and backed by modern dwellings within
In this view, the main part of Tamarindo Beach is in the background, with the entrance to an estuary that curves around behind the beach on the right.
a scattered forest. It faces directly onto the Pacific, with its enormous fetch (wave-generation area), and so benefits from a strong year-round incoming swell, making the beach a popular surfing location. To the north and south of the main beach are two further beaches that together form the Las Baulas National Marine Park. These are important nesting sites for the leatherback turtle from October to March.
OCEAN ENVIRONMENTS
COASTAL SETTING
PACIFIC OCEAN EAST
114
COASTS AND THE SEASHORE
Estuaries and Lagoons ESTUARIES AND COASTAL LAGOONS ARE BOTH
semi-enclosed, coastal bodies of water. An estuary typically connects to the open sea, is quite narrow, and receives a significant input of fresh water from one or more rivers. This fresh water mixes with the salt water to a varying degree, depending on river input and tides. Many estuaries are simply the seaward, tidally affected ends of large rivers. Coastal lagoons are usually linked to the sea only by one or more narrow channels, through which water flows in and out; sometimes these channels open only at high tide.
Estuary Formation Estuaries form in four main ways. First, sea level may rise and flood an existing river valley on a coastal plain, such as in Chesapeake Bay in the US. Second, sea level can rise to flood a glacier-carved valley, forming a fjord. Estuaries formed in this way are deeper than other types, but have shallow sills at their mouths that partially block inflowing seawater. Third, coastal wave action can also create an estuary, by building river a sand spit or bar across the open end of a bay fed by a stream or river (see p.93). Fourth, estuaries result from movement at tectonic faults (lines of weakness) in Earth’s crust, where downward slippage can result in a surface depression. This becomes an estuary if DROWNED RIVER SYSTEM seawater later floods in. retreating glacier-carved glacier
estuary
delta
sill
valley
FORMATION PROCESSES
CONGO RIVER ESTUARY
Formed by flooding of a river valley, this estuary is the world’s second largest (after the Amazon) in terms of discharge rate.
An estuary can form when sea-level rise causes the seaward end of a river valley to flood (top) or inundates a glacier-carved valley to create a fjord (middle), or when a spit extends across a bay (bottom).
debris left by glacier (moraine)
estuary (fjord)
FLOODED GLACIAL VALLEY sand spit bay estuary river longshore current SPIT ACROSS A BAY
OCEAN ENVIRONMENTS
Types of Estuaries The way in which fresh and salt water mixes in an estuary determines its classification. A strong river inflow usually means minimal mixing—the less-dense fresh water flows over the denser salt water, which forms a wedge-shaped intrusion into the bottom of the estuary. This is a salt-wedge (river-dominated) estuary. In partially mixed and fully mixed (tide-dominated) estuaries, there is considerable mixing, producing turbulence and increased salinity in the fresh water. In each case, this is balanced by a strong, tidally influenced influx of salt water from the sea: this influx brings sediments from offshore, which are deposited as mud in the estuary. medium flow of fresh water
minimal mixing of salt and fresh water
strong flow of fresh water
SALT-WEDGE ESTUARY
In a salt-wedge estuary (left), there is a strong flow of fresh river water over a wedge of salt water, with little mixing between the two layers.
fresh water
wedge of sea water
horizontal variation in salinity weak flow of fresh water
outflow to sea
slightly salty water flows out
small tidal countercurrent outflow to sea large tidal countercurrent
fresh water
considerable mixing
PARTIALLY MIXED ESTUARY
large tidal countercurrent
In this type of estuary, there is considerable mixing between fresh and salt water. The saltiness of the water increases with depth in all parts of the lower estuary.
thorough vertical mixing
salt water
FULLY MIXED ESTUARY
In a fully mixed (or tide-dominated) estuary, the fresh and salt water are well-mixed vertically, but there is some horizontal variation in saltiness.
ESTUARIES AND LAGOONS CARVED BY GLACIERS
A fjord is an estuary formed when the sea floods a deep valley originally carved out by a glacier. Norway’s Geiranger Fjord is 12 miles (20 km) long, and reaches a depth of 660 ft (200 m).
115
Estuarine Environments Estuaries are unique coastal environments. They are typically long and funnel-shaped, so tides don’t just rise here—they rush in, creating strong currents and, sometimes, wall-like waves called tidal bores. The high COMMON EUROPEAN OYSTER rate of sedimentation means that mud accumulates, so tidal mudflats and salt marshes (see pp.124–25) or in the tropics, mangrove swamps (see pp.130–31), develop. Despite the effects of tides and currents, the high turbidity that reduces plant photosynthesis, and fluctuations in salinity and temperature, most estuaries are biologically highly productive. This is partly due to the high concentration of nutrients in river water, and because estuaries are well oxygenated. Although only a limited range of organisms, such as mussels, cope with living in estuaries, populations are often huge.
RICH FOOD SOURCE
ESTUARY DWELLER
Estuaries attract waders and other shorebirds because of the high concentrations of small animals (such as worms and shrimp) that live in the mud deposits. These lapwings and an egret are congregating to feed in the Thames estuary, UK.
Various species of starfish tolerate the estuarine environment, where they feed on mussels, crustaceans, and worms. This common starfish is in an estuary in Brittany, France.
Coastal Lagoons Coastal lagoons occur worldwide, and are different from the lagoons found at the centers of coral atolls (see p.152). Calmer and usually shallower than estuaries, most lagoons are connected to the sea by tidal channels. Although fresh water does not usually flow into coastal lagoons, some do receive a significant river inflow. So, as well as saltwater lagoons, there are also some partly, or predominantly, freshwater lagoons. In hot climates, some lagoons are hypersaline (saltier than ocean water), due to high evaporative losses. Although some coastal lagoons are severely polluted, the cleaner ones are often well stocked with fish, crustaceans, and other marine life, and frequently attract large numbers of shorebirds. Some provide feeding or breeding areas for sea turtles and whales.
Matagorda Bay is a lagoon on the coast of Texas, separated from the Gulf of Mexico by a long, narrow peninsula. Two channels, located near the southwest corner of the lagoon, connect it to the gulf.
OCEAN ENVIRONMENTS
LAGOON AND CHANNELS
116
COASTS AND THE SEASHORE ATLANTIC OCEAN NORTHWEST
St. Lawrence Estuary TYPE Salt-wedge (river-dominated) estuary
Approximately 10,000 square miles (25,000 square km)
AREA
LOCATION
Quebec, eastern Canada
The St. Lawrence Estuary is one of the world’s largest estuaries. Some 500 miles (800 km) long, it discharges about 3 million gallons (12 million liters) of water into the Gulf of St. Lawrence each second. The estuary is rich in marine life. In its wide middle and lower reaches, the icy Labrador Current flows 1,000 ft (300 m) below the surface in the opposite direction of the main estuarine flow. In one section, near the mouth of a fjord that branches off the estuary, the current’s nutrientrich waters rise abruptly and mix with warmer waters above. This upwelling of nutrients encourages plankton growth, providing the base of a food chain that involves many species of fish and birds, and a small population of beluga whales. WINTER SCENE
In winter, much of the estuary becomes iced over. A stretch of the estuary is seen here at low tide, shortly after sunrise.
ATLANTIC OCEAN NORTHWEST
Chesapeake Bay TYPE
Partially mixed estuary AREA
3,200 square miles (8,200 square km)
OCEAN ENVIRONMENTS
LOCATION Surrounded by Maryland and parts of eastern Virginia, US
Chesapeake Bay is the largest estuary in the US. Its main course, fed by the Susquehanna River, is over 185 miles (300 km) in length. It has numerous sub-estuaries, and more than 150 rivers and streams drain into it. This body of water was created by sea-level rise drowning the valley of the Susquehanna and its tributaries over the last 15,000 years. Once famous for its seafood, such as oysters, clams, and crabs, the bay is now far less
productive, though it still yields more fish and shellfish than any other estuary in the US. Industrial and farm waste running into the bay causes frequent algal blooms, which block sunlight from parts of its bed. The resulting loss of vegetation has lowered oxygen levels in some areas, severely affecting animal life. The depletion of oysters, which naturally filter water, has had a particularly harmful effect on the bay’s water quality.
BAY BRIDGE
DISCOVERY
A major bridge in the upper bay connects Maryland’s rural eastern shore to its urban western shore.
IMPACT CRATER In the 1990s, drilling of the seabed at Chesapeake Bay led to the discovery of a meteorite impact crater 53 miles (85 km) wide under its southern region. The 35-million-year-old crater helped shape today’s estuary.
SHOCKED QUARTZ
Evidence for the crater included the discovery of grains of shocked quartz, which forms when intense pressure alters its crystalline structure.
MAIN CHANNEL FLOWING THROUGH DELTA ATLANTIC OCEAN WEST
Mississippi Estuary TYPE
Salt-wedge (river-dominated) estuary AREA
25 square miles (60 square km) Southeastern Mississippi Delta, southeastern Louisiana, US
LOCATION
The Mississippi Estuary is about 30 miles (50 km) long and lies at the seaward end of the Mississippi River, where the river flows through its own delta. The estuary consists of a main channel and several subchannels. Together, these discharge an average of some 4.75 million gallons (18 million liters) of water per second into the Gulf of Mexico. The main channel is a classic example of a salt-wedge estuary—its surface waters contain little salt, but they flow over a wedge of salt water, which extends deep down for several miles up the estuary.
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ATLANTIC OCEAN WEST
Laguna Madre TYPE
Hypersaline coastal lagoon AREA
14,400 square miles (3,660 square km) Southern Texas, US, and northeastern Mexico, along the coast of the Gulf of Mexico
LOCATION
The Laguna Madre is a shallow lagoon in two distinct parts extending about 285 miles (456 km) along the coast of the Gulf of Mexico. Its northern part, in Texas, is separated from the Gulf by a long, thin barrier island, Padre Island. The southern part, in Mexico, is
similarly cut off by a string of barrier islands. The entire lagoon connects with the Gulf only via a few narrow channels, and it is less than 3 ft (1 m) deep in most parts. It is saltier than seawater because it receives no input of river water and lies in a hot, dry region, leading to high rates of evaporation. Seagrass meadows and several species of crustaceans and fish thrive in the lagoon, which also supports many wintering shorebirds and waterfowl. Threats to the lagoon’s health include dredging, overfishing, and algal blooms. FLY-FISHING FOR REDFISH
The sale of licenses for fly-fishing—for trout and redfish—in the Laguna provides funds for protecting its water quality and wildlife.
ATLANTIC OCEAN SOUTHWEST
Lagoa dos Patos TYPE
Tidal coastal lagoon AREA
3,900 square miles (10,000 square km)
LOCATION
South of the city of Porto Alegre, southern
Brazil
TWO LAGOONS
In this aerial view, Lagoa dos Patos is the pale central area. Below it, the darker Mirim Lagoon extends to the Brazil–Uruguay border.
OCEAN ENVIRONMENTS
Lagoa dos Patos (“Lagoon of Ducks”) is the world’s largest coastal lagoon. Its name is said to have been given to it by Jesuit settlers in the 16th century, who bred waterfowl on its shores. It is a shallow, tidal body of water, 155 miles (250 km) long and up to 35 miles (56 km) wide. A sand bar separates it from the Atlantic, with which it connects at its southern end via a short, narrow channel that disgorges a large plume of sediment into the ocean. Marine animals use this channel to access the lagoon; sea turtles are found in the lagoon in spring and summer. At its northern end, the lagoon receives an inflow of fresh water from the Guaíba Estuary, formed from the confluence of the Rio Jacui and three smaller rivers. Along its inner side are a number of distinctive wavelike “cusps” that have been caused by the accumulation and erosion of
sediments driven by tidal action and winds. The salinity of the lagoon varies. It consists mainly of fresh water at times of high rainfall, but there is considerable saltwater intrusion at its southern end at times of drought. Lagoa dos Patos is one of Brazil’s most vital fishing grounds. However, run-off from rice fields and pastureland, industrial effluents, and increasing population have led to concerns for the lagoon’s ecosystem.
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COASTS AND THE SEASHORE ATLANTIC OCEAN SOUTHWEST
Amazon Estuary TYPE Salt-wedge (river-dominated) estuary
Approximately 7,800 square miles (20,000 square km)
AREA
LOCATION
Northern Brazil HUMAN IMPACT
The Amazon Estuary is a stretch of the Amazon River that extends more than 190 miles (300 km) inland from the river’s mouth to an area southwest of the city of Macapà.Varying in width from 15 to 190 miles (25 to 300 km), the estuary is partly filled by numerous low-lying, forested islands. The Amazon Estuary has by far the largest water output of any estuary in the world, discharging an average of 45 million gallons (200 million liters) per second into the Atlantic. The sheer magnitude of this discharge means that, almost uniquely among estuaries, there is very little saltwater intrusion into it. Instead, nearly all of the mixing between the river’s discharge and seawater occurs outside the estuary, on an area of continental shelf. Despite the relative lack of seawater intrusion, the whole estuary is significantly affected by twice-daily tides, which cause inundation (by river water) of most of the islands in the estuary.
POROROCA SURF Tidal bores, locally called pororocas, occur on large spring tides in several of northern Brazil’s river estuaries. Some of these bores attain heights of 10 ft (3 m) and can be surfed for several miles. This sport is rather hazardous, however, because the waters through which the pororocas surge are home to dangerous snakes, fish, and crocodiles.
MARAJO ISLAND
The Amazon Estuary is so enormous that the biggest of the forested islands lying within it, Marajo Island, has its own river system.
ATLANTIC OCEAN SOUTHWEST
River Plate TYPE
Salt-wedge (river-dominated) estuary AREA
13,500 square miles (35,000 square km)
OCEAN ENVIRONMENTS
LOCATION On the Argentina–Uruguay border, east of Buenos Aires and southwest of Montevideo
SATELLITE VIEW
The main flow of river water over the sediments on the estuary bed is visible here, as well as the Paraná River at top left and the Uruguay River at top center.
The Plate River, or Rio de la Plata, is not a river but a large, funnel-shaped estuary formed by the confluence of the rivers Uruguay and Paraná. These rivers and their tributaries drain about one-fifth of the land area of South America. At 180 miles (290 km) long and 136 miles (220 km) wide at its mouth, the Plate discharges about 6.5 million gallons (25 million liters) of water per second into the Atlantic Ocean. As well as transporting this
vast amount of water, the estuary receives about 2 billion cubic feet (57 million cubic meters) of silt each year from its input rivers. This mud accumulates in great shoals, so that the water depth in most of the estuary is less than 10 ft (3 m). Constant dredging is therefore needed to maintain deep-water channels to the ports of Buenos Aires, which lies near the head of the estuary, and Montevideo, which is close to its mouth. Surface salinity
varies uniformly through the estuary, from close to zero in its upper parts to a value just below average ocean salinity near its mouth. Deep down, a wedge of salt water penetrates deep into the estuary. Biologically, the Plate is highly productive, yielding large annual masses of plankton, which support large numbers of fish and dense beds of clams. It is also a habitat for the La Plata Dolphin, an endangered long-beaked species of river dolphin.
119 ATLANTIC OCEAN NORTHEAST
Curonian Lagoon TYPE
Freshwater coastal lagoon AREA
610 square miles (1,580 square km) On the Baltic Sea coasts of Lithuania and the Kaliningrad Oblast (part of Russia)
LOCATION
The Curonian Lagoon is a nontidal lagoon on the southeastern edge of the Baltic Sea, with an average depth of just 12 ft (3.8 m). The Neman River flows into the lagoon’s northern (Lithuanian) section, which discharges into the Baltic via a narrow channel, the Klaipeda Strait. While most of the lagoon consists of fresh water, seawater sometimes enters its northern part via the Klaipeda Strait following storms. In the past, the lagoon has suffered heavy
pollution from sewage and industrial effluents, but attempts are now being made to address this problem. The lagoon is separated from the Baltic by the narrow, curved Curonian Spit, which is 60 miles (98 km) long. The spit is notable for its mature pinewoods and drifting barchans (sand dunes), some reaching a height of 200 ft (60 m), which extend for 20 miles (31 km) along the spit. The sandy beaches on the spit, together with vistas over the lagoon, woods, and drifting dunes, make it a tourist attraction, and in 2000 the entire spit was designated a UNESCO World Heritage site. DUNES AND LAGOON
This quiet corner of the northern part of the lagoon is backed by the Curonian Spit’s high dunes. Migrating birds use the lagoon and nearby Neman Delta for vital rest breaks.
ATLANTIC OCEAN NORTHEAST
Hardanger Fjord TYPE Highly stratified estuary; fjord
Approximately 290 square miles (700 square km)
AREA
LOCATION
Southeast of Bergen, southwestern
Norway
READS ISLAND
This low-lying island, in the upper part of the estuary, is a breeding ground for avocets and other rare birds and is managed as a nature reserve. The view here is looking downstream.
Humber Estuary Fully mixed (tide-dominated) estuary
TYPE
Approximately 80 square miles (200 square km)
AREA
West and southeast of Kingston-uponHull, eastern England, UK
LOCATION
This large estuary on Great Britain’s eastern coastline is formed from the confluence of the Ouse and Trent rivers. It discharges about 66,000
ATLANTIC OCEAN NORTHEAST
Eastern Scheldt Estuary TYPE
Former estuary, now a sea-arm AREA
140 square miles (365 square km) Southwest of Rotterdam, southwestern Netherlands
LOCATION
The Eastern Scheldt Estuary is a tidal body of water 25 miles (40 km) long, with a salinity similar to that of seawater. Since the late 1980s, it has
The fjord’s narrow upper parts are fed by several spectacular waterfalls, such as the Vøringsfossen, which freefalls 600 ft (182 m).
Hardanger Fjord was formed about 10,000 years ago, when a large glacier that had carved out and occupied a deep U-shaped valley in the area began to melt and retreat. As it did so, seawater flooded into the valley to create the fjord. Today, the fjord continues to receive a large input of fresh water from glacier melt. Throughout much of its length, the fjord is stratified into a lower layer of salt water, which moves into the fjord during flood tide, and an upper layer of fresher water that flows outward to the sea on the ebb tide. been cut off from its input of fresh water from the Scheldt River by dams, leading to its reclassification as a sea-arm rather than an estuary. It has also been defended against seawater flooding by a storm-surge barrier (see p.104). This was originally to have been a fixed dam to prevent any ingress of seawater at all, but there were fears that, with a dam of this type, the estuary would gradually lose its salinity, producing an adverse effect on its fauna and flora—in particular, there were concerns that it would end the large-scale mussel and oyster farming in the area and degrade the tidal flats and salt marshes that form an important habitat for birds. The government of the Netherlands therefore commissioned a movable barrier, the construction of which was completed in 1986. STORM BARRIER GATES
The gates are usually raised, allowing tidal water in and out of the Eastern Scheldt Estuary. They are lowered about twice a year, during stormy weather.
OCEAN ENVIRONMENTS
ATLANTIC OCEAN NORTHEAST
gallons (250,000 liters) of water per second into the North Sea, the largest input from any British river into this sea. After the end of the last ice age, when sea levels were much lower, the Humber was a river that flowed up to 30 miles (50 km) past the present coastline before reaching the sea. About 3.6 million cubic feet (100,000 cubic meters) of sediment are deposited in the estuary every year, mainly from offshore by tidal action. Shifting shoals formed by this sediment can obstruct shipping. The estuary’s intertidal areas are productive ecosystems that support a wide range of mollusks, worms, crustaceans, and other invertebrates. These are vital sources of food for birds, especially waders. The estuary also supports a colony of gray seals, and many lampreys pass through it every year.
Like all fjords, the Hardanger Fjord in Norway is much deeper than a typical coastal-plain estuary, with a maximum depth of some 2,600 ft (800 m). Near its mouth is a sill just 500 ft (150 m) deep. At 114 miles (183 km) long, it is the third-longest fjord in the world.
UPPER FJORD
120 ATLANTIC OCEAN NORTHEAST
Gironde Estuary Fully mixed (tide-dominated) estuary
TYPE
Approximately 200 square miles (500 square km)
AREA
LOCATION
North of Bordeaux, western France
The Gironde Estuary, formed by the confluence of the Garonne and Dordogne rivers, is the largest estuary in Europe at almost 50 miles (80 km) long and up to 7 miles (11 km) wide. The estuary’s average discharge rate into the Atlantic is 265,000 gallons (1 million liters) per second. It has a large tidal range, of up to 16 ft (5 m) during periods of spring tide, and the strong tidal currents in the estuary, as well as numerous sand banks, tend to
hamper navigation. One of the Gironde’s most impressive features is its tidal bore—a large, wall-like wave at the leading edge of the incoming tide—known locally as the Mascaret. Occurring with each flood tide at the time of spring tides (that is, twice daily for a few days every two weeks), the bore surges from the Gironde upstream into its narrower tributaries. On the Garonne, the Mascaret sometimes forms a barreling wave, which can reach a height of 5 ft (1.5 m) and tends to break and reform. The Gironde is an important artery of the Bordeaux wine region and a rich source of eels and a wide variety of shellfish, which feature on local restaurant menus. Wild sturgeon (the source of caviar) were once also plentiful in the estuary, and although their numbers have declined due to overfishing, they are still farmed in small numbers.
THE MASCARET
When it reaches the Dordogne River, the Mascaret, or Gironde tidal bore, turns into a series of waves, which may travel up to 20 miles (30 km) upstream.
ATLANTIC OCEAN EAST
Venetian Lagoon TYPE
Saltwater coastal lagoon AREA
210 square miles (550 square km) LOCATION
On the Adriatic coast of northeastern Italy
The Venetian Lagoon is a very shallow, crescent-shaped coastal lagoon off the northern part of the Adriatic Sea. It is the largest Italian wetland and a major Mediterranean coastal ecosystem.
In addition to Venice, which sits on a small island at the center, the lagoon contains many other islands, most of which were marshy but have now been drained. Its average depth is just 28 inches (70 cm), so most boats cross the lagoon only via dredged navigation channels, and four-fifths of its area consists of salt marshes and mudflats. It takes in both riverine fresh water and seawater, and its tides have a range of up to 3 ft (1 m). During periods of spring tide,Venice is regularly flooded (see p.90), although engineering works designed to prevent this are due to be completed in 2016. Land subsidence and rising sea levels also pose a major
threat to the city and its art treasures. Marine life in the lagoon includes many species of fish (from anchovies to eels, mullet, and sea bass) and invertebrates. Seabirds, waterfowl, and waders proliferate on the many uninhabited islands. Efforts are now being made to reduce industrial and agricultural pollution, including attempts to capture pollutants by means of shrubs planted along the edges of the lagoon. WATERY GEM
In the center of this photograph, taken from the International Space Station, is the fish-shaped main island of Venice. Below it is one of the lagoon’s three protective barrier islands.
JAMES ISLAND
ATLANTIC OCEAN EAST
Gambia Estuary Salt-wedge (river-dominated) estuary
TYPE
Approximately 400 square miles (1,000 square km)
OCEAN ENVIRONMENTS
AREA
LOCATION
East of Banjul, Gambia, West Africa
The Gambia Estuary is the western half of the Gambia River, which runs 700 miles (1,130 km) through West Africa. The estuary is tidal throughout and discharges about 528,000 gallons (2 million liters) per second into the Atlantic during the rainy season, but only 528 gallons (2,000 liters) in the dry season. It contains abundant stocks of fish and shellfish, including catfish, barracuda, and shrimp. Kunta Kinteh Island, or James Island, some 20 miles (30 km) from the estuary’s mouth, was formerly a slave-collection point and is now a UNESCO World Heritage Site.
ESTUARIES AND LAGOONS ATLANTIC OCEAN EAST
Ebrié Lagoon TYPE
Coastal lagoon of variable salinity AREA
200 square miles (520 square km) LOCATION
West of Abidjan, Ivory Coast, West Africa
The Ebrié Lagoon is one of three long, narrow lagoons that line the shores of the West African state of Ivory Coast. With a length of 62 miles (120 km) and an average width of 21/2 miles (4 km), it is the largest lagoon in West Africa. Its average depth is 16 ft (5 m). Near its
eastern end, it connects to the Atlantic via a narrow artificial channel, the Vridi Canal, opened in 1951. Abidjan, the largest city in Ivory Coast, stands on several converging peninsulas and islands in an eastern part of the lagoon; other communities situated on or in the lagoon include the town of Dabou and the village of Tiagba (see below). The Komoé River provides the main input of fresh water. In winter the lagoon becomes salty, but it turns to fresh water during the summer rainy season. Levels of pollution in the lagoon have been high for some years due to dumping of refuse and discharge of untreated industrial effluents and sewage from the nearby urban areas.
INDIAN OCEAN NORTH
Kerala Backwaters TYPE Chain of coastal saltwater lagoons
Approximately 400 square miles (1,000 square km)
AREA
TIAGBA VILLAGE
Southeast of Cochin, Kerala State, southwestern India
LOCATION
In the village of Tiagba, on the outskirts of a small island in the Ebrié Lagoon, the buildings are raised up on wooden piles.
The backwaters of Kerala in southern India are a labyrinth of lagoons and small lakes, linked by 900 miles (1,500 km) of canals. The lagoons are
LAKES AND LAGOON
In this satellite view, the Coorong Lagoon is the narrow blue strip behind the yellow sand dunes. Above are the lakes Alexandrina (left) and Albert (right).
Coorong Lagoon TYPE
Saltwater coastal lagoon AREA
80 square miles (200 square km) Southeast of Adelaide on the southeastern coast of South Australia
LOCATION
The Coorong Lagoon is a wetland that lies close to the coast of South Australia. It is famous as a haven for birds, ranging from swans and pelicans
to ducks, cranes, ibis, terns, geese, and waders such as sandpipers and stilts. The lagoon is separated from the Great Australian Bight (considered part of the Indian Ocean) by the Younghusband Peninsula, a narrow spit of land covered by sand dunes and scrubby vegetation. The lagoon is about 93 miles (150 km) long, with a width that varies from 3 miles (5 km) to just 330 ft (100 m). At its northwestern end, the lagoon meets the outflow from Australia’s largest river, the Murray, after the river has passed through Lake Alexandrina. In this region, called the Murray Mouth, both river and lagoon meet the sea,
and the Coorong can receive both fresh and salty water. The lagoon was once freely connected to the lake, from which it received a much larger supply of fresh water. In 1940, however, barrages were built between the lagoon and the lake to prevent seawater from reaching the lake and the lower reaches of the Murray River. The salinity of the lagoon’s waters increases naturally with distance from the sea due to evaporative losses. However, reduced water flows from the Murray, due to a combination of barrage construction and extraction of water for irrigation projects, has caused a gradual further increase in salinity throughout the
VEMBANAD LAKE
Vembanad, the largest Kerala coastal lagoon, is listed as a Wetland of International Importance under the Ramsar Convention.
shielded from the sea by low barrier islands and spits that formed across the mouths of the many rivers flowing down from the surrounding hills. During the summer monsoon rains, the lagoons overflow and discharge sediments into the sea, but toward the end of the rains, the seawater rushes in, altering salinity levels. The aquatic life in the backwaters, which includes crabs, frogs, otters, and turtles, is well adapted to this seasonal variation.
PELICANS IN DECLINE The Coorong is home to a large breeding colony of Australian Pelicans, which inhabit a string of islands in the center of the lagoon. Since the 1980s, however, their numbers have fallen significantly due to reduced flows of fresh water into the Coorong from the Murray River. The resultant higher salt levels in the lagoon have reduced the growth of an aquatic weed that is a major part of the food chain.
AUSTRALIAN PELICANS
This pelican, one of seven species worldwide, is widespread in Australia, where it lives on freshwater, brackish, and saltwater wetlands.
lagoon. There is ample evidence that this has adversely affected the lagoon’s ecosystem. In particular, several species of plants have become less abundant or disappeared, many fish species have declined, and migratory bird numbers have fallen. Further, the reduced flow from the Murray may result in the eventual closure of the channel joining the lagoon to the ocean, which would prevent migration of fish and other animals between the two.
OCEAN ENVIRONMENTS
INDIAN OCEAN SOUTHEAST
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COASTS AND THE SEASHORE PACIFIC OCEAN WEST
Pearl River Estuary TYPE
Salt-wedge (river-dominated) estuary AREA
1,200 square km (450 square miles) Northwest of Hong Kong, Guangdong, southeastern China
LOCATION
INDIAN OCEAN SOUTHEAST
Northern Spencer Gulf Estuary TYPE
Inverse estuary AREA
Approximately 5,000 square km (2,000 square miles) LOCATION
Northwest of Adelaide, South Australia
The estuary in the north of Australia’s Spencer Gulf is classified as an inverse estuary, owing to its unusual pattern of salt distribution and water circulation.
DEEP GULF
Spencer Gulf is the larger wedge-shaped coastal indent visible in this satellite image. The desert around its head helps produce the estuary’s unusual circulation pattern.
In a reverse of the usual pattern, this estuary’s waters become saltier towards it head, away from its mouth. This is because its head is surrounded by hot desert and loses more water to evaporation than enters it from rivers. The head’s high salinity means that it draws in from the mouth ocean water of lower salinity than the water drawn in by a typical estuary. The estuary is surrounded by extensive tidal flats, seagrass banks, and mangroves.
The bell-shaped Pearl River Estuary receives and carries most of the outflow from the Pearl River, the common name for a complex system of rivers in the southern Chinese province of Guangdong. The estuary is nearly 60km (37 miles) long, and its width increases from 20km (12 miles) at its head to about 50km (30 miles) at its mouth. To the north and west of the estuary is a delta, formed from the confluence of the Xi Jiang and other rivers of the Pearl River system. Together, these rivers discharge an average of GUANGZHOU
Formerly known as Canton, this large and busy port city lies on a northerly extension of the Pearl River Estuary.
PACIFIC OCEAN WEST
Yangtze Estuary TYPE
Partially mixed estuary AREA
2,500 square km (1,000 square miles)
OCEAN ENVIRONMENTS
LOCATION
SHANGHAI YANGTZE RIVER BRIDGE
This bridge across the Yangtze Estuary, situated very close to its mouth, is 10km (6 miles) long. It opened in 2009.
10 million litres (2.2 million gallons) of water per second into the South China Sea. Mostly less than 9m (30ft) deep, but containing some deeper dredged channels, the Pearl River Estuary has a tidal range of 1–2m (3–6ft). It drains water from one of the most densely urbanized regions in the world and is severely polluted as a result of billions of tonnes of sewage and industrial effluent entering it each year. One result of this has been the increasing occurrence of algal blooms that threaten local fishing. Pollution is also a threat to a dwindling population of Chinese White Dolphins (less than 1,000) that live in the estuary. Only since 2008 have efforts begun to reduce pollution through the building of more water treatment plants.
Northwest of Shanghai, eastern China
The Yangtze Estuary is the lower, tide-affected part of the Yangtze (or Changjiang) – the longest river in Asia and the third longest in the world. The estuary occupies 700km (430 miles) of the river’s 6,300-km (3,900-mile) length. Near its mouth, it splits into three smaller rivers and numerous streams that run through a delta. Here, silt deposition continually creates new land, which is used for agriculture. The estuary carries an average of 30 million litres (6.6 million gallons) of water per second into the East China Sea; its average depth is 7m (23ft), and the average tidal range at its mouth is 2.7m (9ft). It supports large numbers of fish and birds, although fish stocks have declined over the past 20 years due to overfishing and pollution. A species of river dolphin that used to live in the estuary, the Baiji or Yangtze River Dolphin, is now thought to be extinct. In winter, salt water intrudes a significant distance upstream, making the water unfit for drinking and irrigation. Recently, this intrusion has occurred more frequently due to reduced river flow – a reduction exacerbated by the Three Gorges Dam project further upstream. Reduced flows have worsened the acute water shortage in Shanghai on the estuary’s southern shore, as well as affected the dispersion and dilution of pollutants around the estuary. Silt deposition in the delta is also likely to fall, reducing the rate of new land creation.
ESTUARIES AND LAGOONS PACIFIC OCEAN SOUTHWEST
Doubtful Sound TYPE
Highly stratified estuary; fiord AREA
70 square km (30 square miles) West of Dunedin, southwestern South Island, New Zealand
LOCATION
Doubtful Sound is one of 14 major fiords that were formed 15,000 years ago in a scenic part of New Zealand’s South Island. Some 40km (25 miles) long and opening onto the Tasman Sea, it is surrounded by steep hills from which hundreds of small waterfalls descend during the rainy season. Its name originated in 1770 during the first voyage to New Zealand by the English explorer Captain James Cook (1728–79). He called the fiord Doubtful Harbour
because he was sceptical of being able to sail out again if he entered it. Doubtful Sound is the second-longest and the deepest of the New Zealand fiords, with a maximum depth of 421m (1,380ft). It receives fresh water from a hydroelectric power station at its head and from a huge 6,000m (236in) of rainfall annually. Like all fiords, it contains fresh water in its top few metres and a much denser, colder, saltier layer below. There is little mixing between the two. Doubtful Sound is home to Bottlenose Dolphins, New Zealand Fur Seals, and many species of fish, starfish, sponges, and sea anemones. SOUND VIEW
This view of the head of Doubtful Sound, looking towards the open ocean, is from the hills of the south-central region of New Zealand’s South Island.
PACIFIC OCEAN NORTHEAST
San Francisco Bay AREA
LOCATION
Central California, western USA
San Francisco Bay, the largest estuary on North America’s west coast, consists of four smaller, interconnected bays. One of these, Suisun Bay, receives fresh water drained from about 40 per cent of California’s land area. This water flows into San Pablo Bay and then Central Bay, where it mixes with salt water that has entered at depth from the Pacific Ocean via the Golden Gate channel. From Central Bay, there is little flow of fresh water to the largest body of water, South San Francisco Bay, but there is
Laguna San Ignacio TYPE
Hypersaline coastal lagoon AREA
360 square km (140 square miles) On the Pacific coast of the Baja California Peninsula, Mexico, southeast of Mexicali
LOCATION
OAKLAND BAY BRIDGE
Thick fog surrounds the lower half of the San Francisco–Oakland Bay Bridge, one of five bridges that cross the bay.
TYPE
Partially mixed tectonic estuary 1,200 square km (460 square miles)
PACIFIC OCEAN EAST
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The Laguna San Ignacio is a coastal lagoon in northwestern Mexico best known as a sanctuary and breeding ground for Pacific Gray Whales. Latin America’s largest wildlife sanctuary, it is also an important feeding habitat for four endangered species of sea turtle. The lagoon, which is 40km (25 miles) long and on average 9km (6 miles) wide, receives only occasional inflows of fresh water, and its evaporative losses are high. Its salinity is therefore
significantly higher at its head than at its mouth, where it connects to the sea. Apart from whale watching, the main human activities in the area are small-scale fisheries and oyster cultivation. In 1993, the lagoon was designated a World Heritage Site. LAGOON BEACH
Waves break on the shore at San Ignacio Lagoon, which is surrounded by a landscape of sparse desert scrub.
some surface outflow of brackish water to the Pacific. San Francisco Bay is a tectonic estuary – one caused by movement at tectonic faults (lines of weakness) in the Earth’s crust, of which there are several in the area, notably the San Andreas Fault. During the past 150 years, human activity has resulted in the loss of 90 per cent of the bay’s surrounding marshy wetland, a greatly reduced flow of fresh water (which has been diverted for agricultural purposes), and contamination by sewage and effluent. Nevertheless, the bay remains an important ecological habitat. Its waters are home to large numbers of economically valuable marine species, such as Dungeness Crab and California Halibut, and millions of geese and ducks annually use the bay as a refuge.
HUMAN IMPACT
WHALE WATCHING
OCEAN ENVIRONMENTS
The Laguna San Ignacio is a popular whale-watching site. Between January and March, large numbers of Gray Whales can be found there. The whales, which often approach boats, use the upper part of the lagoon for giving birth, while the lower lagoon is where males and females look for mates. Females swim with their calves in the middle part of the lagoon.
124
COASTS AND THE SEASHORE
Salt Marshes and Tidal Flats A SALT MARSH IS A VEGETATED AREA OF COAST
AT L A
that is partly flooded by the sea at high tide and completely flooded by the highest spring tides. Many areas of salt marsh are bordered by tidal flats. These are broad areas of mud or sand, mainly without vegetation, that are uncovered at low tide and covered as the PA C I F I C tide rises. Salt marshes and tidal flats are depositories for large OCEAN amounts of organic material, derived from decaying plants and animals. This provides the base for an extensive food chain.
N T
IC
O C
EAN
INDIAN OCEAN
HERN OCEAN SOUT
Formation and Features Tidal flats occur on low-energy sheltered coasts, such as estuaries and enclosed bays, where sediment held in the water settles out and builds up. The most extensive flats occur where there is a high tidal range. Tidal flats may consist either of sand (sandflats) or mud (mudflats), or a mixture of these. Mudflats contain a higher concentration of the decaying remains of dead organisms than sandflats and are also the first stage in the development of salt marshes. These develop on the landward side river delta of mudflats. As various salt-tolerant plants grow, their roots trap sediment and stabilize the mud. As the vegetated flat builds up, different mainland types of plants become established. The result is a salt marsh, consisting of blocks of flat, low-growing vegetated areas of mud, broken up by sinuous channels.
DISTRIBUTION
Salt marshes and tidal flats occur only north of the latitude of 32˚N and south of 38˚S. In latitudes nearer the equator, they are replaced by mangrove swamps. COASTAL SETTING
Salt marshes commonly develop in coastal lagoons or in estuarine areas that are sheltered from the sea by spits or barrier islands. The channels transport salt water, plankton, nutrients, sediment, and plant detritus into and out of the marsh.
estuary mainland
channels
flood delta
BAY OF FUNDY
In this small sub-estuary of Canada’s Bay of Fundy, an area of salt marsh is visible in the background. In the foreground is a broad intertidal area of mud and gravel.
dunes
barrier island
KEY
dunes
salt marsh ebb delta
inlet
lagoon
tidal flats
Zones and Evolution
OCEAN ENVIRONMENTS
Salt marshes have two main zones. The parts flooded by every high tide are called low marsh, while the areas that are only occasionally flooded are termed high marsh. Each zone is colonized by distinct species of salt-tolerant plants. Each species, of which there are many, has developed special mechanisms to deal with the high levels of salt they are exposed to: some possess salt-excreting glands, for instance, while others have storage systems for collecting the salt until they can dilute it with water. Salt marshes and adjoining mudflats usually evolve over time. As sediment builds up, the mud surface in the marsh, the adjoining flats, and the bay or estuary as a whole tends to rise. As it does so, areas of low marsh become high marsh and areas of mudflat are colonized by plants, turning into low marsh.
SEA LAVENDER
Sea lavenders are common high-marsh colonizers. They bloom in summer, producing purple or lavender flowers.
SALT-MARSH CORDGRASS
SALT-MARSH ZONES
Also called smooth cordgrass, this species is the dominant low-marsh plant throughout the Atlantic coast of North America. Stands of this grass grow to 7 ft (2 m) high.
The low marsh is the part flooded once or twice a day at high tide, while the high marsh is the area above the mean high-tide level—it is flooded only occasionally, by the highest spring tides. Each zone has distinctive vegetation. pool
highest spring tide mean high tide upper high marsh mean sea level
lower high marsh
upland
high marsh
low marsh
mudflat
125
ALGAE-COVERED MUDFLATS
Some mudflats, such as these in Alaska, become heavily encrusted with green algae. The algae is often itself colonized by large numbers of tiny marine snails. HUMAN IMPACT
CONSERVING SALT MARSHES Salt marshes are threatened worldwide through being built on, converted to farmland, or even used as waste dumps. Over half of the original salt marshes in the US, for example, have been destroyed. This is regrettable, as salt marshes are valuable wildlife habitats and centers of biodiversity.
Animal Life Measured by the amount of organic matter (the base material for food chains) that they produce, salt marshes are extremely productive habitats. Most of this material comes from decaying plant material. When plants die, they are partially decomposed by bacteria and fungi, and the resulting detritus is consumed by animals such as worms, mussels, snails, crabs, shrimp, and amphipods living in the marsh, and zooplankton living in the salt water. These in turn provide food for larger animals. Salt marshes provide nursery areas for many species of fish, and feeding and nesting sites for birds such as egrets, herons, harriers, and terns. Tidal flats are home to many types of crustaceans, worms, and mollusks, which either feed on the surface or burrow beneath it. These in turn provide food for enormous numbers of wading birds.
A common inhabitant of salt marshes in the USA and parts of east Asia, the Great Egret, and closely related Eastern Great Egret, feeds on small fish, invertebrates, and small mice. MARSH HOUSING DEVELOPMENT
This coastal development in Myrtle Beach, South Carolina, has been built on top of a drained salt marsh. However, the adjoining area of marsh has been carefully preserved.
LUGWORM CASTS
Lugworms live in burrows some 8–16 in (20–40 cm) deep in tidal flats. They feed by taking in sand or mud, digesting any organic matter, and excreting the rest as a cast.
This toad, found in parts of western and northern Europe, inhabits upper salt marsh habitats (just below the high marsh), where it uses shallow ponds to breed.
OCEAN ENVIRONMENTS
GREAT EGRET
NATTERJACK TOAD
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MARSH AT LOW TIDE
Patches of salt marsh surround the basin, together with tidal flats that can extend for up to 3 miles (5 km) from the shore at low tide.
ATLANTIC OCEAN NORTHWEST
Minas Basin TYPE Tidal sandflats and mudflats, and salt marshes
490 square miles (1,250 square km)
AREA
LOCATION Eastern part of Bay of Fundy, Nova Scotia, Canada
The Minas Basin is a semi-enclosed inlet of the Bay of Fundy. It consists of a triangular area of tidal mudflats and sandflats, surrounded by patches of salt marsh, most of which have been diked and drained for agriculture. Twice a day, the sea fills and empties the basin, rising and falling by over 40 ft (12 m), which is the largest tidal range in the world. No other coastal marine area has such a large proportion of its floor exposed at low
tide. Sediments in the basin, which are brought in and deposited by tides, range from coarse sand to fine silt and clay. The tidal flats formed by these sediments contain high densities of a marine amphipod, the Bay of Fundy mud-shrimp, which provides food for huge numbers of migrating shorebirds,
ATLANTIC OCEAN NORTHWEST
Cape Cod Salt Marshes TYPE
Salt marshes AREA
30 square miles (80 square km)
OCEAN ENVIRONMENTS
LOCATION
Cape Cod, eastern Massachusetts, US
Salt marshes are the dominant type of coastal wetland around Cape Cod, although about a third of the region’s marshes have been lost or severely degraded within the past 100 years. These salt marshes occur behind barrier beaches or spits and within estuarine systems, and have developed over the past 3,000 years in response to sea-level rise. They mainly RACE POINT
A typical area of salt marsh can be seen here behind dunes at Race Point, at the northern extreme of Cape Cod.
including sandpipers and plovers. The numbers peak from July to October, and for some species exceed 1 percent of the world population. SEMIPALMATED SANDPIPER
Half a million semipalmated sandpipers stop off in the Minas Basin each year on their way from North America’s Arctic regions to South America.
consist of high marsh, where the dominant plant species is saltmeadow cordgrass, with some scattered areas of low intertidal marsh, dominated by smooth cordgrass. The low marsh areas are flooded twice daily and the high marsh twice a month, during the highest spring tides. The largest individual marsh is the Great Salt Marsh to the west of the town of Barnstable. With deep channels running through it, this is a popular area to explore by kayak. The marshes around Cape Cod serve as a breeding and foraging habitat for a diversity of brackish and freshwater animals. Among these are two rare and protected bird species, the northern harrier and least tern, and two endangered reptiles, the diamond-backed terrapin and eastern box turtle. Restoring degraded salt marshes on Cape Cod is regarded as a top priority for many regional and national conservation organizations. Restoration will allow these wetlands to regain their function as a barrier protecting the coastline from storm surges and as a natural sponge that filters pollutants and excess nutrients from the water runoff in the region.
127 ATLANTIC OCEAN NORTHWEST
South Carolina Low Country TYPE
Salt marshes and tidal mudflats AREA
630 square miles (1,600 square km) South Carolina coast, southwest and northeast of Charleston, US
LOCATION
The Low Country contains one of the most extensive systems of salt marsh and tidal flats in the United States. Its size results from the broad, gently sloping, sandy coast of the US eastern seaboard, coupled with a moderately high tidal range of 5–7 ft (1.5–2 m).
Each day, two high tides inundate a vast area of the coastal zone, maintaining a system of channels, creeks, and rivers. The influence of both fresh and salt water here results in some diverse ecological communities. Smooth cordgrass is the dominant grass in the lower marshes, where the ground stays wet and muddy as a result of the tides. From late spring to fall, darker dead-looking sections of a grass called needle rush can also be seen. These two grasses are replaced toward higher ground by sea oxeye and the similar but taller marsh elder. In the lower marshes and the bordering tidal flats, mud snails, crabs, shrimp, worms, and other tiny inhabitants burrow into the mud, while attached and clinging to the stalks of the grasses are ribbed mussels and marsh periwinkles. Among the fish living in the silty tidal wash are croaker, menhaden, and mullet. Birds living here include marsh wrens and clapper rails. CORDGRASS MEADOWS
A tidal channel weaves its way through stands of smooth cordgrass, the dominant plant species in the lower marsh areas.
ATLANTIC OCEAN NORTHEAST
Morecambe Bay Tidal mudflats and sandflats, and salt marshes
TYPE
120 square miles (310 square km)
AREA
LOCATION
Northwest England, UK
MORECAMBE MUDFLATS
The ebbing tide reveals half of the bay’s total area as undulating expanses of mud and sand, meandering channels, and tidal pools.
The edges of the Wadden Sea are a mosaic of marsh patches broken up by shallow tidal channels.
ATLANTIC OCEAN NORTHEAST
Wadden Sea TYPE Tidal mudflats and sandflats, salt marshes, and islands
4,000 square miles (10,000 square km)
AREA
North Sea coast from Esbjerg, Denmark, along northern Germany, to Den Helder, Netherlands
LOCATION
The Wadden Sea is an extensive body of shallow water and associated tidal flats, salt marshes, and low-lying islands in northwestern Europe. Straddling the shores of Denmark, Germany, and the
Netherlands, the Wadden Sea has been formed by storm surges and sea-level rise inundating an area of coast, combined with the deposition of fine silt by rivers. It is an important nursery for North Sea fish species such as plaice and common sole, and its extensive mudflats are home to a number of mollusks and worms. The salt marshes provide a habitat for more than 1,500 species of insects and are important feeding and breeding grounds for many species of birds. Unfortunately, these marshes are threatened by intensive farming, industrial development, and climate change. In 2009, parts of the region were declared a UNESCO World Heritage Site.
HUMAN IMPACT
COCKLING Morecambe Bay has many rich cockle beds. The cocklers use planks of wood called jumbos to soften the sand, which helps draw the cockles to the surface. Because of the fast-moving tides, cockling has to be carried out with an eye on safety. In February 2004, a total of 23 Chinese migrant workers drowned after being cut off by the tides.
OCEAN ENVIRONMENTS
Formed from the confluence of five estuaries, those of the Kent, Keer, Leven, Lune, and Wyre rivers, Morecambe Bay is the largest continuous area of tidal flats in the UK. Broad, shallow, and funnel-shaped, the bay has a large tidal range, of up to 35 ft (10.5 m). During periods of spring tides, the sea can ebb as far as 7 miles (12 km) back from the high-water mark. The flood tide comes up the bay faster than a person can run, and parts of the bay are also affected by quicksand, posing dangers for anyone who does not know the area well. The bay’s extensive mudflats support a rich and diverse range of invertebrate animals, including cockles and mussels, snails, shrimp, and lugworms, as well as one of the largest populations of shorebirds in the UK. The bay regularly hosts 170,000 wintering waders, with several species present in internationally significant numbers, including oystercatchers, curlews, dunlins, and knots. The tidal flats are surrounded by extensive salt marshes, which make up about 5 percent of the total salt marsh in
the UK and support a number of rare plants. Much of this marsh area is grazed by sheep and cattle. The bay is an important location for commercial fishing; the fish species most commonly caught include bass, cod, whitebait, and plaice. However, Morecambe Bay has not escaped the problems of pollution common to many coastal areas of northwestern Europe. Oil, chemicals, and plastic are among the more common pollutants of this ecosystem.
SALT-MARSH MOSAIC
128
COASTS AND THE SEASHORE ATLANTIC OCEAN NORTHEAST
The Wash TYPE
Salt marshes, tidal sandflats, and mudflats AREA
100 square miles (250 square km) LOCATION
Northeast of Peterborough, England, UK
comprise the largest single area of this habitat in Britain and are growing in extent. The main plant species making up these salt marshes, which are traditionally used as grazing lands by farmers, are cordgrass and glasswort in roughly equal amounts. The Wash is one of the most important sites in the UK for wild birds, its sheltered tidal flats providing a vast feeding ground for migrating birds, such as geese, ducks, and waders.
These come to spend the winter in the Wash in huge numbers, with an average total of about 300,000 birds, from as far away as Greenland and Siberia. In addition, the Wash is an important breeding area for common terns and a feeding area for marsh harriers. It has been declared a Special Protection Area (SPA) under EU law. In 2000, parts of the artificial coastal defenses on the western side of the Wash were deliberately breached
to increase the area of salt marsh in the region. This has taken pressure off other nearby sea defenses, because the newly establishing area of salt marsh soaks up wave energy, acting as a natural sea defense. This is a relatively novel approach to coastal management that employs “soft engineering” techniques to defend against the erosive power of the sea. It also has the added environmental advantage of providing additional habitat for wildlife.
The Wash is a large, square-mouthed, shallow estuary on the eastern coast of England, surrounded by extensive areas of tidal sandflats, some mudflats, and salt marshes. It is fed by four main rivers: the Great Ouse, Nene, Welland, and Witham. The sandflats of the Wash range from extensive fine sands to drying banks of coarse sand and are home to large communities of bivalve mollusks, crustaceans, and polychaete worms. The extensive salt marshes TERRINGTON MARSHES
Located close to the mouth of the Nene River, these marshes form part of the Wash National Nature Reserve.
HUMAN IMPACT
SALT HARVEST
EDGE OF THE MARSHES
OCEAN ENVIRONMENTS
The dominant plant species in the nonexploited areas of salt marsh, such as at La Turballe in the northern part of the marshes, include sea-blite, cordgrass, and glasswort. ATLANTIC OCEAN NORTHEAST
Guérande Salt Marshes TYPE Salt marshes, artificial salt pans, and tidal mudflats
7 square miles (50 square km)
AREA
LOCATION
Northwest of St. Nazaire, Atlantic coast
of France
The region of salt marshes close to the medieval town of Guérande is most famous for its salt production but is also a noted ecological site, important for its role as a feeding and resting site
for large numbers of birds. The salt marshes came to exist in their present state through a combination of geology, climatic factors, and human intervention. Around the coast near Guérande, a system of spits and coastal dunes developed thousands of years ago, cutting off an area of shallow water, which was nevertheless subject to tides—seawater could flow in through two inlets in the dune belts. Over the centuries, marshes and tidal flats developed in this basin. During the past 1,000 years or so, these have been artificially converted into a mosaic of salt pans, separated by clay walls, although some areas remain unexploited. During the flood tide, seawater is allowed to flow through
channels into the pans, and during the warm summer months, when the rate of evaporation is high, sea salt is skimmed from the surface of the pans by an army of salt-farmers (paludiers). The areas of marsh surrounding the salt pans are made up of various salt-tolerant plants. More than 70 different species of birds nest and breed in the area, and many species spend the winter here in large numbers. For many years, the salt-farmers and the French ornithological society, the LPO, have jointly organized exhibitions and guided tours in the Guérande Salt Marshes, which are themed on the economics of salt production, the ecology of the marshes, and their need for protection.
The Guérande region has had salt pans for over 1,000 years. Today, about 300 salt-farmers work in the area, one of the few places in France where salt continues to be produced in a traditional manual way. The average annual harvest is about 10,000 tons of natural, mineral-rich sea salt, which is sold unrefined, with nothing added and nothing removed. The salt has a light gray color because of its content of fine clay from the salt pans.
129 PACIFIC OCEAN NORTHWEST
PACIFIC OCEAN NORTHWEST
Saemangeum Wetlands
Yatsu-Higata Tidal Flat
TYPE
TYPE
Mudflats, sandflats, and salt marshes
Tidal mudflat AREA
AREA
1/6 square mile (0.4 square km)
155 square miles (400 square km) LOCATION
South of Seoul, on the west coast of South Korea
LOCATION
Situated at the confluence of the Mangyeung and Dongjin river estuaries, on South Korea’s Yellow Sea coast, the Saemangeum Wetlands is a shorebird staging site of great importance. Its tidal flats and shallows support many bird species, some of which are considered to be globally threatened. In 2010, the status of this
Yatsu-Higata is a tiny rectangular mudflat at the northern end of Tokyo Bay, and is unusual because it is almost completely surrounded by a dense urban area. Once open shoreline, Yatsu-Higata now sits 3/5 mile (1 km) inland. Twice daily, it experiences a tidal inflow and outflow of water from Tokyo Bay via two concrete channels. When the tide comes in, the mudflat fills with about 3 ft (1 m) of water. When it flows out, a variety of resident and migrant shorebirds congregate to feed on the lugworms, crabs, and other marine animals that live within the fine silt that remains. Yatsu-Higata is an important stopover point for migrating birds flying from Siberia to Australia and Southeast Asia.
SPOON-BILLED SANDPIPER
This extremely rare species is one of the shorebirds most threatened by the reclamation project.
PACIFIC OCEAN NORTHEAST
Alaskan Mudflats TYPE
Tidal mudflats AREA
4,000 square miles (10,000 square km)
Various coastal inlets of southern and western Alaska, US
LOCATION
Narashino City, at the northern part of Tokyo Bay, Japan
wetland—as well as the thousands of migratory birds that depend on it as a key feeding area— came under threat due to the completion of a 22-mile- (33-km-) long sea wall at the mouth of the two estuaries. The sea wall is part of a project to turn the Many areas on the coast of southern and western Alaska are fringed by mudflats that appear at low tide. They are formed of a finely ground silt that in some areas is several hundred yards deep. This silt has originated from the action of Alaska’s numerous glaciers, which have been grinding away at the surrounding mountains for thousands of years. As these glaciers melt, the silt is carried to the coast in meltwater and deposited as sediment
LOW TIDE AT DONGJIN ESTUARY
The area around the estuary consists of tidal flats and scattered salt marsh intersected by channels that fill at high tide.
wetland into dry land for industrial or agricultural use, together with a freshwater reservoir. The project is going ahead despite the fears of conservation groups that it will result in irreversible environmental damage. when it reaches the sea. Because tidal ranges around Alaska are generally high, the total area of mudflats exposed at low tide is huge. These mudflats are an important stopover for migrating shorebirds. Various species of burrowing worms and bivalve mollusks are an important source of food for these waders and for the waterfowl that feed on the mudflats through the winter. Harbor seals also use the mudflats as rest areas.
Brown bears are occasional visitors to some Alaskan mudflats, where they dig for Pacific razor clams buried in the mud. They probably find the clams by looking for the small holes they leave on the surface as they burrow down. Extracting them is tricky, since when disturbed, they burrow down further. ALASKAN BROWN BEAR
This large adult bear is digging on the coast of Katmai National Park, at the eastern end of the Alaskan Peninsula.
DRYING MUDFLATS
These mudflats are at the edge of a large delta on the southwest coast of Alaska, formed by the Yukon and Kuskokwim rivers.
OCEAN ENVIRONMENTS
DIGGING FOR CLAMS
The mudflats are dangerous for human visitors, because in some areas they behave like quicksand. Even mud that at first seems firm enough to support a person may in reality be treacherous. A number of people have become stuck and some have even drowned.
130
COASTS AND THE SEASHORE
Mangrove Swamps A MANGROVE SWAMP IS A COLLECTION
of salt-tolerant evergreen trees, thriving in an intertidal environment in the tropics or subtropics. Mangrove swamps line about eight percent of the world’s coastlines, where they filter pollutants AERIAL ROOTS from river runoff and help prevent the silting up of adjacent Many mangrove species have aerial roots. These marine habitats. They also protect coastlines against erosion and prop the tree up and take in oxygen, which is provide a home for fish, invertebrates, and many other animals.
Formation
AT L A
N T
IC O C
Mangrove swamps develop in coastal areas protected from direct wave action. These areas often fringe estuaries and coastal lagoons (see p. 114). Most mangroves develop in fine muds or sandy sediments that form in these environments. As the lower parts of the mangrove roots develop in the sediment, aerial roots form a tangled network above it. This traps silt and other DISTRIBUTION Mangrove swamps occur only material carried there by rivers and tides. between latitudes 32˚N and 38˚S. PA C I F I C Land is built up, and then colonized by Salt marshes and tidal flats (see OCEAN p.124) replace them elsewhere. other types of vegetation.
usually not available in the mud that most mangroves grow in.
EAN
INDIAN OCEAN
OCEAN ENVIRONMENTS
ERN OCEAN SOUTH
Plants Some 54 species of trees and shrubs are classified as “true” mangroves, occurring only in mangrove habitats. Each has evolved special adaptations to the conditions they grow in, such as salty water. For example, some mangroves can excrete salt in their leaves. On most mangrove shorelines, there are two or three zones, each dominated by different mangrove species. In the Americas, just four main species are found. The area closest to the sea is dominated by red mangroves. Landward of this are black mangroves—the roots of this and some PNEUMATOPHORES other species develop pencil-like breathing These vertical tubes grow tubes, called pneumatophores. White and up out of the sand or mud button mangroves grow farther landward. The as extensions of horizontal mangrove swamps on the coasts of the rest of roots. When exposed to the tropics contain greater species richness. the air, they take in oxygen.
RED MANGROVE
This mangrove species can grow in deep water by means of its numerous prop roots, which often have a reddish tint. It also has a particularly high salt tolerance.
131
Animal Life
BANDED ARCHERFISH
Mangrove swamps are rich centers of biodiversity. Mangrove trees produce enormous amounts of leaf litter, as well as twigs and bits of bark, which drop into the water. Some of this immediately becomes food for animals such as crabs, but most is broken down by bacteria and fungi, which turn it into food for fish and shrimp. These in turn produce waste, which, along with the even smaller mangrove litter, is consumed by mollusks, amphipods, marine worms, small crustaceans, and brittlestars. Some of these become food for larger fish, and the various fish species provide food for larger animals. Across the world, mangrove swamps are home to an enormous number and MANGROVE diversity of birds and several endangered BRITTLESTAR This scavenger is one species of crocodiles. Other types of of the few echinoderms animals found in great numbers and found in mangrove diversity in mangrove swamps include swamps. It is highly frogs, snakes, insects, and mammals mobile, using its long ranging from swamp rats to tigers. arms to pull itself along.
This little fish inhabits mangrove swamps in the Indian and Pacific oceans. It is known as an archer- fish because it feeds mainly on flying insects, which it knocks out of the air and into the water by spitting at them.
JABIRU STORK
This large stork inhabits mangrove swamps and other wetlands throughout the tropical Americas, feeding on a range of prey, including snakes.
SHRIMP FARMING
SHELTER FROM PREDATORS
Cardinalfish, sheltering here in a mangrove swamp in Papua New Guinea, are one of the many types of small tropical fish that use mangrove roots for protection from predators.
About a quarter of the world’s mangrove swamps have been destroyed since 1980 and have been built on or turned into commercial enterprises such as shrimp farms (including the one shown here, in Vietnam). Unfortunately, intensive shrimp farming often has devastating environmental effects. Typically, the effluent from shrimp ponds pollutes nearby coastal waters, destroying more mangroves as well as coral reefs along the coastline.
OCEAN ENVIRONMENTS
HUMAN IMPACT
132
MANGROVE-LINED CHANNEL
Here, parallel stands of red mangrove line a shallow offshoot channel of Florida Bay in the southern part of the Everglades National Park.
ATLANTIC OCEAN WEST
Everglades PRINCIPAL SPECIES
Red, black, and white Mangroves Mangroves only: 600 square miles (1,500 square km)
AREA
LOCATION
Southwestern Florida, US
Mangroves occupy a large, roughly triangular area at the southwestern tip of southern Florida, where a maze of islands along the coast is intersected by mangrove-lined channels. Here, where the salt water of the Gulf of Mexico and Florida Bay meets fresh water that has traveled from Lake Okeechobee in central Florida, is the largest area of mangrove swamps in North America.
ATLANTIC OCEAN WEST
ATLANTIC OCEAN WEST
Alvarado Mangrove Coast
Sian Ka’an Biosphere Reserve
PRINCIPAL SPECIES
PRINCIPAL SPECIES
Red, white, and black mangroves
Red, black, white, and button Mangroves AREA
200 square miles (500 square km)
OCEAN ENVIRONMENTS
AREA
400 square miles (1,000 square km)
LOCATION To the southwest of Veracruz, Mexico, on southwestern Gulf of Mexico
LOCATION
The Alvarado Mangroves Ecoregion in southern Mexico is an extensive area of mangrove swamps mixed in with other habitats such as reed beds and palm forests. The mangroves grow on flat coastal land interspersed with brackish lagoons fed by several small rivers. The swamps are brimming with life, from rays gliding in the calm waters to snails climbing the mangrove roots, whose tangled network protects many fish and invertebrates from predators. Bird life in and around the swamps includes the keel-billed toucan, reddish egret, wood stork, and several species of herons and kingfishers, while the mammalian inhabitants include spider monkeys and West Indian manatees. Some large areas of mangroves in the region have been destroyed, and those that remain are under pressure from logging, agricultural expansion, oil extraction, and frequent oil spills.
Stretching for 75 miles (120 km) along Mexico’s Caribbean coast, the Sian Ka’an Biosphere Reserve contains a mixture of mangrove swamps, lagoons,
Eastern coast of Yucatán Peninsula, eastern Mexico, 90 miles (150 km) south of Cancún
ANHINGA
This diving bird hunts fish, frogs, and baby alligators in the Everglades mangroves.
The dominant species along the edges of the sea and the numerous channels is the red mangrove—water within the channels is normally stained brown from tannin contained in the leaves of this species. In addition to their role in stabilizing shorelines with and freshwater marshes; it was declared a World Heritage Site by UNESCO in 1987. The mangroves are protected from the energy of the Caribbean Sea by a barrier reef growing along the coast. However, the reserve’s terrestrial part is between 20 and 75 percent flooded, depending on season. Sian Ka’an’s mangrove systems are some of the most biologically productive in the world and their health is critical for the survival of many species in the western Caribbean region. Hidden between the massive mangrove roots live oysters, sponges, sea squirts, sea anemones, hydroids, and crustaceans. Bird species found here include roseate spoonbills, pelicans, greater
their large prop roots, red mangroves are crucial to the Everglades ecosystem, acting as a nursery for many species of fish, as well as shrimp, mussels, sponges, crabs, and other invertebrates. The other principal mangrove species in the Everglades are the black mangrove and white mangrove. Both of these grow closer to the shore than red mangroves, so they are in contact with seawater only at high tide. The Everglades swamps provide a feeding flamingos, jabiru storks, and 15 species of heron. The swamps are also home to West Indian manatees and two endangered crocodiles: the American crocodile and Morelet’s crocodile. The explosion of tourism in the nearby resort of Cancún poses several threats to the area. Unregulated development has increased pollution and altered the distribution and use of water in Sian Ka’an, compromising the health of the mangroves. BOAT TOUR
Because a large part of Sian Ka’an is flooded for much of the year, there are few roads into the area, so much of it can be reached and explored only by boat.
133
CICHLID INVASION
and nesting site for several mammals, including swamp rats, and numerous bird species, such as herons, egrets, gallinules, anhingas, and brown pelicans. While much of the region has an abundant alligator population, the swamps are the sole remaining stronghold in the US for the rare and endangered American crocodile. Also occasionally spotted in the channels between the mangroves are West Indian manatees (sea cows).
The Florida mangroves are sometimes damaged by the hurricanes that hit the region, hurricanes Katrina and Wilma in 2005 being examples. Hurricanes damage mangroves in two ways: strong winds may defoliate them, and storm surges harm them by depositing large quantities of silt on their roots. Fortunately, mangrove forests are resilient ecosystems, and they usually regenerate fully from hurricane damage within a few years.
Since 1983, the Mayan cichlid, an exotic fish species from Central America, has been spreading rapidly through the mangrove swamps and other wetland areas of the Everglades. No one yet knows what effect it may have on the region’s ecosystem. There are worries that it may displace native fish species; alternatively, it could be occupying a new “niche” that no other fish species has filled.
ATLANTIC OCEAN WEST
Belize Coast Mangroves PRINCIPAL SPECIES
Red, black, white, and button mangroves AREA
600 square miles (1,500 square km) Eastern Belize, on the western margins of the Caribbean Sea
LOCATION
rivers and trapping sediment, the mangroves also protect the clarity of the coastal waters, helping the coral reef to survive. Numerous cays—small islands composed largely of coral or sand—along the coast are covered with mangroves and form a habitat for birds. In all, more than 250 bird species share the swamps with West Indian manatees and a variety of reptiles, including boa constrictors, American crocodiles, and iguanas. MANGROVE ROOTS
ATLANTIC OCEAN WEST
Zapata Swamp PRINCIPAL SPECIES
AREA
1,500 square miles (4,000 square km) Western Cuba, 100 miles (160 km) southeast of Havana
LOCATION
The Zapata Swamp is a mosaic of mangrove swamps and freshwater and saltwater marshes that form the largest and best-preserved wetland in the Caribbean. The swamp was designated a Biosphere Reserve in 1999 and forms a vital preserve for Cuban wildlife, a spawning area for commercially valuable fish, and a crucial wintering territory for millions of migratory birds from North America. More than
Large numbers of these colorful birds live in the swamp, feeding off algae, shrimp, mollusks, and insect larvae that inhabit the mud at the bottom of the shallow waters.
900 plant species have been recognized in the swamp, and all but three of the 25 bird species endemic to Cuba breed there. All together, about 170 bird species have been identified in the swamp, including the common black-hawk, the greater flamingo, and the world’s smallest bird, the bee hummingbird. It also contains the remaining few thousand Cuban crocodiles. Mammalian residents include the Cuban hutia, a gopherlike rodent, and the West Indian manatee. The manjuari, or Cuban gar, is an unusual fish found only in the swamp. Adjacent to the swamp is the Bay of Pigs, where millions of land crabs breed each spring.
The mangrove swamps here are a nursery ground for many fish species associated with the huge Belize Barrier Reef. By filtering runoff from
A tangled maze of mangrove roots extends beneath the water’s surface all along this coast, providing refuge for a variety of juvenile fish.
OCEAN ENVIRONMENTS
Red, black, white, and button mangroves
GREATER FLAMINGOS
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COASTS AND THE SEASHORE INDIAN OCEAN WEST
Madagascar Mangroves PRINCIPAL SPECIES Gray, yellow, long-fruited stilt, and large-leafed orange mangroves
1,300 square miles (3,300 square km)
AREA
Scattered areas around the coast of Madagascar, off the eastern coast of Africa LOCATION
INDIAN OCEAN NORTH
Pichavaram Mangrove Wetland
Mangroves occur in a wide range of environmental conditions on Madagascar, fostered by a high tidal range, extensive low-lying coastal areas, and a constant supply of fresh river water, which brings a high silt load. They occupy about 600 miles (1,000 km) of the island’s coastline and are often associated with coral reefs, which protect them from ocean swell. The mangroves, in turn, capture river sediment that otherwise would threaten both reefs and seagrass beds. Up to nine different mangrove species
INDIAN OCEAN NORTH
Sundarbans Mangrove Forest
PRINCIPAL SPECIES Gray, milky, stilted, small-fruited orange, and yellow mangroves
PRINCIPAL SPECIES
5 square miles (12 square km)
AREA
Sundri, milky mangrove, yellow mangrove, Indian mangrove, keora 3,200 square miles (8,000 square km)
OCEAN ENVIRONMENTS
AREA
LOCATION Tamil Nadu, southeastern India, 90 miles (150 km) south of Chennai
LOCATION
Southwestern Bangladesh and northeastern India, between Calcutta and Chittagong
The Pichavaram Mangrove Wetland lies on a delta between the Vellar and Coleroon estuaries in southeastern India. It consists of a number of small and large mangrove-covered islets intersected by numerous channels and creeks. Fishing villages, croplands, and aquaculture ponds surround the area. This small, carefully preserved wetland is thought to have saved many lives during the 2004 Indian Ocean tsunami. When the tsunami struck, six villages that were physically protected by the mangroves incurred no damage, while other, unprotected villages were totally devastated. The wetland may have reduced the tsunami’s impact partly by slowing the onward rush of the sea through frictional effects and partly by absorbing water into its numerous canals and creeks.
This forest, a World Heritage Site since 1997, is the largest continuous mangrove ecosystem in the world. It is part of a
have been recorded in Madagascar, although only six are widespread. Several of Madagascar’s endemic birds, including the Madagascar heron, Madagascar teal, and Madagascar fish-eagle, use the mangroves and associated wetland habitats. Dugongs (relatives of manatees) glide through the waters, feeding on sea grasses, while huge quantities of invertebrates and fish swim freely among the fingerlike roots of the mangroves. These provide an abundance of food for animals such as the Nile crocodile,
sharks, and aquatic and wading birds, such as herons, spoonbills, and egrets. Many of the fish and bird species here are found nowhere else in the world. Unfortunately, the mangroves are threatened by urban development, overfishing, and the development of land for rice and shrimp farming. MANGROVE MAZE
This area of coastal mangroves, bisected by numerous channels, is located on the northeastern coast of Madagascar, at the mouth of the Ambodibonara River.
huge delta formed by sediments from the Ganges, Brahmaputra, and Meghna rivers.The region contains thousands of mangrove-covered islands intersected by an intricate network of waterways.The Bengal tiger swims here from island to island, hunting prey such as spotted deer and wild boar. Other inhabitants include fishing cats, rhesus macaque monkeys, water monitor lizards, hermit crabs, the Gangetic dolphin, and various sharks and rays. Habitat destruction threatens this region: more than half of the original mangroves have been cut down.
GAVIAL One extremely endangered inhabitant of the wetlands and rivers of Bangladesh is the gavial, a crocodilian. Once quite common in the Sundarbans, their numbers have dwindled due to accidental capture in fishing nets and other factors. Gavials are probably heading for regional extinction, although captive breeding programs in India and Nepal aim to save the species.
SATELLITE VIEW
In this satellite view of part of the Ganges– Brahmaputra–Meghna delta, the Sundarbans Mangroves form the area that appears dark red. On the right is the Bay of Bengal.
135 PACIFIC OCEAN WEST
Kinabatangan Mangroves PRINCIPAL SPECIES Stilt mangrove, long-fruited stilt mangrove, gray mangrove, nipa palm
400 square miles (1,000 square km)
AREA
LOCATION
Southeast of Sandakan, eastern Sabah,
Malaysia
Mangrove swamps occupy a coastal region of the Kinabatangan River delta, within eastern Sabah in the northern part of the island of Borneo. The mangrove swamps in this area form a complex mosaic with other types of lowland forest (including palm forest) and open reed marsh. They are home to dozens of species of saltwater fish, invertebrates such as shrimp and crabs, otters, and some 200 species of birds including various species of fish eagle, egret, kingfisher, and heron.
Irrawaddy dolphins are also occasionally spotted in the region, while other spectacular inhabitants include Borneo’s indigenous proboscis monkey and the saltwater crocodile (the world’s largest crocodile species), which was almost hunted to extinction but whose numbers are now recovering. Over the past 30 years, there has been extensive clearance of mangroves in the Kinabatangan delta for purposes of timber and charcoal production. The mangroves have either been replaced by oil palms or the cleared land has been developed for shrimp farming. Inevitably, the wildlife has suffered, but the government of Sabah is now engaged in a large-scale mangrove replanting operation. MANGROVE MONKEY
A female proboscis monkey, able to both swim and walk upright, is seen here with an infant, leaping across a waterway in the Kinabatangan mangroves. Her long tail helps to stabilize her movement through the air.
PACIFIC OCEAN EAST
Darien Mangroves
AERIAL ROOTS AT LOW TIDE
The mangroves are anchored in the soft mud by a dense network of roots that also provide a habitat for many animals.
PRINCIPAL SPECIES
Red, black, button, white, mora, and tea mangroves 360 square miles (900 square km)
AREA
Southeast of Panama City on the Pacific coast of eastern Panama
LOCATION
The Darien mangrove swamps lie around estuaries in eastern Panama in the Darien National Park, adjacent to the Gulf of Panama. Here, the roots of mangroves create a haven for mollusks, crustaceans, and many fish species. Shrimp are particularly abundant— the larvae hatch offshore, migrate to the mangrove “nursery” for a few months, and then return to sea as adults. Some of the mangrove swamps in this region have been converted to shrimp ponds or farmland. Other threats include urbanization and pollution. BLACK MANGROVE PACIFIC OCEAN WEST
New Guinea Mangroves
4,000 square miles (10,000 square km)
AREA
Scattered areas around the island of New Guinea in the western Pacific
LOCATION
Mangrove swamps occur in extensive stretches on New Guinea’s coastline. The longest and deepest stretches are found on the south side of the island, around the mouths of large rivers such as the Digul, Fly, and Kikori rivers. Mangrove communities here are the most diverse in the world—more than 30 different species of mangroves have
relatively low. Two endemic species of bats and a species of monitor lizard are found here. Ten bird species are endemic, including the New Guinea flightless rail, two species of lory, the Papuan swiftlet, red-breasted paradisekingfisher, and red-billed brush-turkey. Although largely intact, the mangrove regions in the western part of New Guinea have recently come under threat of pollution from the rapidly expanding oil and gas industries. SEAHORSE
This small seahorse is adopting the yellow color of fallen mangrove leaves.
These black mangroves are in the Punta Patiño Nature Reserve, a private reserve owned by a nonprofit environmental group.
OCEAN ENVIRONMENTS
PRINCIPAL SPECIES Gray, long-fruited stilt, tall-stilted, and cannonball mangroves
been found in a single swamp—and they form a vital habitat for a variety of animals living on the water’s edge. Underwater, over 200 different fish species, ranging from cardinal fish and mangrove jacks to seahorses and anchovies, have been recorded in either their adult or juvenile stages. Mudskippers (species of fish that can leave the water and climb trees), snails, and crabs climb the mangrove roots, while saltwater crocodiles patrol the channels between the mangrove stands. Although there are many species of fish and mangrove in these swamps, terrestrial animal diversity is
NEW GUINEA MANGROVES
This young saltwater crocodile is feeding among mangrove roots. Fully grown, this species is the largest of all crocodiles, growing up to 23 ft (7 m) long. Despite its name, it prefers fresh water, and adults compete fiercely for control of prime channels in swamps, often forcing juveniles into marginal rivers or out to the open sea.
THE SHALLOW SEAS that cover the
continental shelves around Earth’s landmasses nurture an extraordinary diversity of life. Energy from the Sun and nutrients from the land and sea ensure good conditions for plant growth, on which all marine life depends. The Moon is also a key player. Its gravitational pull drives the tides, which uncover the seashores each day, creating tidal currents that distribute plant nutrients and bring food to waiting animals. Each area of seabed provides a specific habitat for marine life that has adapted to the local conditions. The shallow seas comprise those parts of the oceans with which we are most familiar; yet we are only just beginning to understand the complexity of life there and its importance to the overall health of the planet.
SHALLOW SEAS CORAL REEFS
From polar seas to the tropics, the reflective surface of the sea hides a realm populated with unfamiliar life forms. Here in the tropics, animals look like plants, and plants hide inside the tissues of corals.
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SHALLOW SEAS
Continental Shelves CONTINENTAL SHELVES ARE ESSENTIALLY
the flooded edges of continents, inundated by sea-level rise after the last ice age. The shelf seabed is now approximately 600ft (200m) below the surface, and its width varies, occasionally extending to hundreds of miles. The shelf seabed and water quality are influenced by land processes. Rivers bring fresh water and nutrients, making shelf waters very productive ecologically, while river-borne material settles on the seabed as sediment. The continental shelf has a huge diversity of marine life and habitats, but it is also the area of the sea that suffers most from pollutants.
Fertile Fringes
SHALLOW SEAWEED
Seaweeds grow best on shallow, sunlit rocks, thrive in strong water movement, and provide food and shelter for many small animals.
The coastal fringes have the greatest diversity of life in the oceans. Light penetration is highly variable, from turbid basins to clear tropical waters. In many places, enough light reaches the shallow sea bed for good growth of photosynthetic organisms. Seaweeds, seagrasses, and phytoplankton thrive here, fed by solar energy, nutrients from land, and sediments stirred up by winds and currents. The coastal fringes are much more productive than the open oceans. Combined with diverse habitats, this results in complex marine communities, making rich feeding and nursery grounds for animals from deeper water. In higher latitudes, seasonal variations in the Sun’s strength stimulate an annual cycle of plankton and seaweed growth. In the tropics, where seasons are less pronounced, seagrasses and seaweeds grow year-round.
DISCOVERY
FIORDS Fiords are deep, sheltered sea inlets originally gouged out by glaciers and then flooded by the sea. They often extend many kilometres inland and are made up of deep basins, separated from the open sea by shallow sills. This basin-and-sill structure has a huge influence on marine life. In this sheltered environment, still, dark salt water lies beneath peaty fresh water. This mimics the marine conditions off the continental shelf, and animals normally confined to much deeper water, such as cold-water corals, inhabit water shallow enough for divers to explore.
Productive Plains Much of the continental shelf is covered with deep sediments. Sand, gravel, and pebbles are deposited in shallow water, while fine mud is carried into deeper water offshore. An important part of shelf sediments is biogenic (made from the remains of living organisms). It consists of carbonates (chemical compounds containing carbon) derived from, for example, coral skeletons, and microscopic plankton. At first sight, sediment plains appear barren. However, many different animals live hidden beneath the surface, either permanently or emerging from burrows and tubes to feed and reproduce. Shifting sand and gravel is a difficult place to live, but more stable sediments occur on deeper sea beds.Varying particle size makes it suitable for constructing burrows and tubes, and it can contain huge numbers of animals, providing a rich food source. These animal communities are all sustained by plankton falling from the continental-shelf surface waters, and by the products of decomposition of seagrasses and seaweeds.
SEDIMENT PREDATORS
Fish and starfish are top predators on sediments, eating the many different animals on the surface or buried beneath. Fish catch a wide range of creatures, while starfish capture slower-moving prey.
OCEAN ENVIRONMENTS
Shelf Fisheries The waters and sea bed of the continental shelf support most of the world’s major fisheries. In coastal waters, there is planktonic food for larvae and cover for juveniles, and this is where QUEEN SCALLOP 90 percent of the world’s total seawater Scallops feed by filtering seawater, catch reproduces. Demersal fish (living and can be collected by diving, or on or just above the seabed) such as cod farmed, with no damage to the and haddock feed on seabed life. Pelagic marine environment. (open water) shoaling fish such as sardines and herring feed on zooplankton, and are important food for larger fish such as mackerel and sharks, as well as for cetaceans and seabirds. Commercially important invertebrates such as shrimp are caught in shelf waters. Worldwide, JUVENILE SHELTER coastal communities are sustained by These baby cod are feeding small-scale, inshore fisheries, which in horse mussel beds, before moving offshore as adults. catch a wide range of marine life.
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SHELF DEPOSITS
The Mississippi River flows into the sea through a network of channels. As its silt-laden waters reach the sea, sediments fall out along the continental shelf.
Geology of the Continental Shelf
COASTAL POLLUTION For many years, coastal seas have been used as a convenient dump for human waste. Even the most remote seashores are now littered with plastic. More insidious is invisible pollution: nutrients and pathogens from sewage; heavy metals, organohalogens, and other toxins from industrial and agricultural effluents; radioactive waste from power stations; and hydrocarbons from effluents, oil spills, and other sources.
DREDGED TREASURE
Metals such as gold, tin, rare earth elements, and aggregates for the building industry are extracted by dredging continental shelves.
OCEAN ENVIRONMENTS
HUMAN IMPACT
Shelf deposits can be extremely thick. For example, those off eastern North America are up to 9 miles (15km) deep, and have been accumulating and compacting for millions of years. A cross-section here reveals ancient sediments other than those deposited by rivers and glaciers, including carbonates, evaporites, and volcanic materials. Carbonates are largely produced by marine life in shallow tropical seas. Evaporites are salts resulting from seawater evaporation in shallow basins or on arid coastlines. Evaporite deposits create domes in overlying sedimentary rocks, trapping oil and gas.
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SHALLOW SEAS
Rocky Seabeds
ROCKY SLOPE
FROM THE WARM TROPICS TO COLD POLAR SEAS, many
distinctive communities of marine life develop on the rocky floors of shallow seas. Underwater rocks provide points of attachment for both seaweeds and marine animals and are often covered with life. Seaweeds thrive in the sunlit shallows and provide a sheltered environment for animal communities. Firmly attached animals extend arms and tentacles to catch planktonic food from water currents, or pump water through their bodies to filter out nutrients. Mobile animals graze seaweeds or prey on fixed animals or each other. The life on a rocky reef depends on many environmental factors.
These underwater rocks in British Columbia, Canada, are covered with marine life. A sunstar and leather star search for prey among pink soft corals and sponges, while urchins graze below.
The Seaweed Zone Seaweeds rely on sunlight for growth, and thrive only on the shallowest rocks. The depth in which they can grow depends on water clarity, from a few yards in turbid seas, to more than 330 ft (100 m) in the clearest waters. In colder waters, huge forests of kelp and other large brown seaweeds dominate the shallows, with smaller seaweeds in deeper water. Large seaweeds are often scarce on rocks in the tropics—instead, the Sun’s energy is harnessed by tiny unicellular algae inside coral tissues. Seaweeds harbor a plethora of associated animals. Some live permanently in the seaweed zone, while others use it as a breeding ground or nursery before moving into deeper water. BALLAN WRASSE FOOD SOURCE
Energy from sunlight captured by seaweeds is used by grazing animals. Here, green seaweeds cover rocks in Orkney, Scotland.
OCEAN ENVIRONMENTS
ROCK GRAZERS Sea urchins are highly successful marine invertebrates, well defended by sharp spines. They graze the seabed, eating virtually everything except hard-shelled animals and coralline seaweed crusts. They have a profound effect on seabed communities. If urchins are abundant, they can seriously reduce the diversity of life on the seabed, leaving urchin “barrens.” Conversely, where urchins are sparse, they can increase diversity, by clearing spaces for new life to settle.
In summer, adult ballan wrasses lay eggs in nests built of seaweed, secured in rock crevices. Young wrasses are often patterned, providing camouflage.
Animal-dominated Deeps In deeper water, light levels are too low for most seaweeds, although encrusting red seaweeds need little light and grow farther down. Much of the plantlike growth in deeper water actually consists of fixed animals, which are most abundant in places with strong tidal currents. For mobile animals living here, the seabed is a minefield of toxic substances, released by fixed animals to deter predators. Below 160 ft (50 m), water movement from waves is much less, and fragile animals such as sponges and sea fans can grow to a large size. Here, and in places more sheltered from water movement, a smothering layer of fine silt continually settles on the rock surfaces, restricting the animal life to forms that can hold themselves above the rock or can remove the silt. On the most heavily silted rocks, animals may grow only on vertical or overhanging surfaces. ROCKY-BOTTOM PREDATOR
Stonefish have a textured skin and irregular shape, making them difficult to spot. A huge mouth engulfs prey, while the dorsal spines contain venom that can be fatal.
protruding eye used when hiding in sediment
dorsal spines with poison glands camouflage skin color and texture
large mouth tail fin
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Vertical Rock Underwater cliffs are often more heavily colonized with invertebrates than gently sloping rocks. In shallow water exposed to strong waves, various mobile seabed animals, particularly grazing sea urchins and predatory starfish, find it harder to cling to vertical and overhanging surfaces, and are knocked off by waves in rough weather.Vertical walls receive less sunlight, and are harder places for seaweed spores to settle, so there is less competition from seaweeds here than on horizontal rock. At sheltered sites, upward-facing rock is often covered with silt, and has few animals, but vertical and overhanging rock, by contrast, is silt-free and may have abundant life. Ledges and crevices in underwater cliffs provide safe refuges for fish and crustaceans.
JEWEL ANEMONES
Multicolored jewel anemones carpet vertical, wave-exposed rocks, with tentacles outstretched to catch food from the currents.
Crevices and Caves Irregularities in underwater rock features can provide additional habitats for marine life. Crevices and small caves provide shelter for nocturnal fish that hide during the day and are active at night. Elongated fish are well shaped to live in crevices, while fish that are active by day need holes to hide in at night and when predators approach. Deep, dead-ended caves contain a range of habitats, from sunlit, wave-exposed entrances to dark, still inner waters and sheltered sediments. Shrimp and squat lobsters occupy cave ledges, while animals that actively pump water to feed live in the quiet water inside the cave and coat the walls. Flashlight fish hiding in caves during the day signal to each other with light produced by bacteria in organs beneath their eyes. Small crevices are important because they form a refuge for small animals from sea urchins. TAKING REFUGE
The flattened body of this spiny squat lobster enables it to retreat far into narrow crevices if threatened, and the spines help to wedge it in small spaces.
Storms and Scour
CORALLINE ALGAE
Like a coating of hard pink paint, encrusting coralline algae can withstand considerable scouring from nearby sand and pebbles.
OCEAN ENVIRONMENTS
Shallow rocky reefs take the full force of waves during storms, but rock-living animals and seaweeds on open, exposed coasts are firmly attached and are generally well-adapted to cope with pounding waves. Larger seaweeds and animals will be torn from shallower rocks, KEELWORMS making space for new life to settle, while many Keelworms have a hard, seaweeds and colonial animals can regrow from calcareous shell that protects holdfasts or basal parts. However, few animals or their bodies from sand scour. plants survive on rolling boulders or on bedrock scoured by nearby sand and pebbles. Where rock meets sand, there is often a band of bare, sandblasted rock. Just above, tough-shelled animals such as keelworms survive, together with patches of hard encrusting calcareous red seaweeds. Above this, fast-growing colonial animals such as sponges and barnacles can colonize in the intervals between storms.
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SHALLOW SEAS
Sandy Sea Beds MOST OF THE CONTINENTAL SHELF
is covered with thick sediments, accumulated from millennia of land and coast erosion. The calcareous remains of marine life are continually added to the mix. Unlike deep-sea sediments (see pp.180-81), shelf sediments are stirred up by waves during storms, resuspending nutrients and profoundly affecting marine life and productivity. Sediments are largely the domain of animals, as seagrasses and seaweeds grow only in limited, shallow areas. Buried beneath the surface of a sandy sea bed, there may be vast numbers of animals hiding from, or waiting for, prey.
Gravel and Sand
coarse bristles (chaetae) on sides
The coarsest sediments from coastal and land erosion are usually deposited inshore by rivers and glaciers as they enter the sea. Frequently shifted by waves and tides, clean, coarse sand and gravel make a difficult habitat; typical inhabitants include tough-shelled molluscs, sea cucumbers, burrowing urchins, and crabs. A wider range of organisms live in the more stable sand and gravel, where purple-pink beds of maerl can be found. This unattached, calcareous seaweed SANDY HABITAT is made up of coral-like nodules. The open structure of live A marine segmented maerl twiglets is ideal for sheltering tiny animals, newly settled worm, the Sea Mouse from the plankton, while the dead maerl gravel underneath supports lives in muddy sand. burrowing animals. Beds of seagrass and green seaweeds thrive in shallow sand, harbouring a wide range of life. Embedded shells and stones provide anchors for various seaweed species. Many fish have adapted to life on sandy sea beds, the most familiar being flatfish. Shallow-water anglerfish wave their fishing lures to tempt prey within striking distance of their huge mouths, while garden eels live permanently in sand burrows, partly emerging to eat plankton. Sand eels and cleaver wrasse dive into the sand to avoid predators. GRAVEL DWELLER
This Flame Shell lives in a nest of gravel, pebbles, and shells. It pumps seawater through the nest, extracting food with its sticky, acidic tentacles.
OCEAN ENVIRONMENTS
Soft Mud In sheltered waters in enclosed bays, estuaries, and fiords, and in the deeper parts of the continental shelf, the finest particles of sediment settle as soft mud. Easily stirred up, the fine particles smother newly settled larvae and clog gills. There is little oxygen just below the mud surface, so buried animals must find ways to obtain oxygen from seawater. Despite these challenges, mud can be very productive. Bacteria and diatoms are often abundant on the mud surface, providing food for hoovering animals such as echiuran worms. Stable burrows are more easily built in mud than in sand or gravel. Animals such as sea pens and burrowing anemones anchor themselves in the mud, raising sticky ANCHORED IN MUD polyps and tentacles This sea pen’s branches to catch the raining are covered with small plankton or to ensnare a polyps that feed on the plankton. passing fish or crustacean.
felt-like dorsal chaetae
EXPLOITING SANDY BEDS
Stingrays are among the many animals that hide in the sand of the sea bed; this Southern Stingray does so both to escape predators and to ambush prey.
Mixed Sediments Most sediments on the continental shelf are a mix of coarse and fine materials. An important part of these are calcareous fragments, derived from hard-shelled animals. Mixed sediments offer a wider range of building materials for tubes and burrows than sand or mud and are easier to traverse, so a far greater variety of animals live here. Seaweeds and hydroids cover the bed, attached to shells and pebbles.Visible life includes tube worms, brittlestars, and burrowing anemones; most of these withdraw into the sediment if threatened. Below the surface, hidden animals, including bivalves LIFE ON THE SEDIMENT and crustaceans, provide a rich Its mouth fringed by tentacles, source of food for animals that this half-buried sea cucumber (left) and a hermit crab inhabit can find and excavate it, such these mixed sediments. as starfish, crabs, and rays.
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SEA-BED STABILIZERS This sea bed owes its luxuriant growth, including hydroids, soft corals, and brittlestars, to the many Flame Shells and Horse Mussels hidden under the surface. These molluscs bind the shifting sediments with strong threads, creating a stable, complex surface that many other animals can colonize. Flame Shell nests join together to form extensive reefs, with holes for water exchange, so many other organisms can live inside and beneath the nests.
DISCOVERY
SEA WRECKS
Beneath the Surface
SEDENTARY HABIT
This Norway Lobster
lives in a U-shaped Wave-disturbed sand and gravel creates a mobile, with two well-oxygenated environment. Animals that live here, burrow exits and is mainly such as crustaceans, and echinoderms, move through nocturnal. the shifting sand without building permanent homes. Animals that disturb sediments in this way, or by ingesting and defaecating it, are called bioturbators and are important recyclers of nutrients. Less-disturbed sediments are inhabited by sediment stabilizers. These sedentary animals, many living in permanent burrows or tubes, can cope with oxygen depletion and being covered over. Some strengthen their burrows by lining them with substances such as mucus and draw in seawater to supply food and oxygen. Others filter seawater or hoover sediment by extending their siphons to the surface. Microscopic creatures (the meiofauna) live in between the sand grains.
OCEAN ENVIRONMENTS
The complex shape and hard surfaces of shipwrecks such as the one shown here (the Eagle, off Florida) attract sedentary invertebrates and fish. A new wreck may take some time to become colonized, depending on the material from which it is made. Small hydroids, barnacles, and keelworms often settle first, paving the way for other animals and seaweeds to grow on their hard shells. Filter feeders thrive in enhanced currents on the super-structure, while the spaces inside offer hiding places for fish and octopuses.
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SHALLOW SEAS
Seagrass Beds and Kelp Forests SEAGRASS BEDS AND KELP FORESTS
are very different habitats, but both are highly productive and contribute significantly to the total primary production of inshore waters. Seagrasses are the only fully marine flowering plants. They thrive in shallow, sunlit water on sheltered, sandy seabeds, primarily in warm water. Kelps are large brown seaweeds that grow as dense forests on rocks of the lower shore and subtidal zone, preferring cold water. Both of these ecosystems have a complex structure and provide shelter for a COLD-WATER KELP Kelps are large brown wide range of associated animals and seaweeds, seaweeds that live mainly in shallow subtidal zones. some of them found nowhere else.
Seagrass Beds
SEAGRASS MEADOWS
Seagrass meadows help to protect shallow sandy seabeds against erosion. NATURAL CAMOUFLAGE
This greater pipefish’s elongated shape and drab color make it hard to spot among seagrass leaves.
Seagrasses are the only flowering plants (angiosperms) that live entirely in the sea, and they grow best in shallow, sandy lagoons or enclosed bays, where the water clarity is good. They are also tolerant of variable salinity. Unlike seaweeds, seagrasses have roots, which they use to absorb nutrients from within the sediments, thus recycling nutrients that would otherwise be locked up below the surface. Their intertwined rhizomes and roots help to stabilize the sand, protecting against erosion and encouraging the buildup of sediments. The productivity and complex physical structure of seagrasses attract a considerable diversity of associated species, some of which are only found in seagrass beds. A variety of seaweeds and sedentary animals, including species of hydroids, bryozoans, and ascidians, grow on the leaves. Seagrasses are also a critically important food for animals such as manatees, dugongs, green turtles, and many aquatic birds.
OCEAN ENVIRONMENTS
Kelp Forests The term “kelp” was originally used to refer to the residue resulting from burning brown seaweeds, which was used in soap-making. It is now used more generally to refer to the many kinds of large brown seaweeds of the order Laminariales. Kelp forests grow best in colder waters, on shallow rocks with good water movement. The top DISTRIBUTION MAP Seagrass beds flourish in the edge of some kelp beds is visible at the lowest tides. tropics, while kelp forests Kelps grow densely on rock slopes down to around in cold, nutrient-rich 30–70 ft (10–20 m) deep, depending on water clarity. In thrive waters, extending into the deeper water, there is less light for photosynthesis and polar regions. kelps grow more sparsely; in most coastal waters they kelp forests cannot survive below 80 ft (25 m). In exceptionally seagrass beds clear water, kelps can grow at 160 ft (50 m). Many kelp species have gas-filled floats, which hold the fronds up to the light and away from grazers. Within the kelp forest, waves are subdued and many organisms live in its shelter. Although kelp habitats support rich marine communities, only about 10 percent of kelp is eaten directly by COASTAL DEFENSES animals; the rest enters A band of giant kelp can help to protect the food chain as coasts from severe detritus or dissolved storms by absorbing wave energy. organic matter.
HUMAN IMPACT
ENDANGERED GRAZERS Seagrasses are the primary food of green turtles, and the only food of manatees and dugongs. Globally, these animals are now endangered or vulnerable, threatened by the destruction of their feeding grounds. The coastal areas in which seagrass beds are found are often vulnerable to pollution. Runoff of nutrients and sediments from land affects water clarity, and is probably the biggest threat worldwide.
SEAGRASS BEDS AND KELP FORESTS
Kelp Communities
Nurseries and Refuges
Many kelps are treelike in shape, with a branched holdfast for attachment and a long stem (stipe), sometimes with floats, supporting a palmlike frond. This makes a kelp forest a multilayered environment in which different organisms live at different levels. Small spaces in the holdfast can KELP ANEMONE harbor hundreds of small animals from predators. This large anemone is unusually mobile, Some kelps have rough stipes covered with red and crawls or drifts up onto seaweed fronds to catch floating prey. seaweeds, although sea urchins and limpets may graze these in calm weather and in deeper water. Actively growing kelp fronds exude slime, which deters most animals from settling, but as growth slows later in the season, the fronds may become covered with a few species, particularly bryozoans, hydroids, and tube worms. These animals reduce the light reaching the fronds, and some kelps shed their fronds to get rid of unwanted settlers before growing new ones. The sea floor beneath the kelps may be covered with marine growth, or relatively barren if heavily grazed by sea urchins.
Seagrass beds and kelp forests are important refuges for young fish that need to hide from predators until they reach maturity. Many fish, such as the lumpsucker and swell shark, do not live among seagrasses or kelps as adults, but come into these habitats to spawn, giving their young a greater chance of survival. Small fish need small prey, and they find an abundance of food in the form of tiny worms, crustaceans, and mollusks among the seagrasses and in the sediment beneath, or in the undergrowth of kelp forests. These young fish are often unlike their parents, usually camouflaged in shades of green and brown to avoid detection. Some herbivorous fish from surrounding reefs come into seagrass beds only at night. Seagrass beds are important nurseries for some commercial invertebrates, including shrimp and cuttlefish.
BLUE-RAYED LIMPET
147
LUMPSUCKER
This baby lumpsucker is very vulnerable. However, it is well camouflaged on kelp fronds, to which it attaches itself with a sucker.
At the end of the growing season, these limpets move down into the holdfast to avoid being discarded with the old frond.
OCEAN ENVIRONMENTS
DENSE KELP FOREST Giant kelp is the world’s biggest seaweed. Its stipes can be more than 100 ft (30 m) long, and it can grow as fast as 20 in (50 cm) per day.
148
SHALLOW SEAS ATLANTIC OCEAN WEST
ATLANTIC OCEAN NORTHEAST
Laguna de Términos COASTAL TYPE
Sound of Barra
Shallow
COASTAL TYPE Island chain with sounds
lagoon WATER TYPE
Tropical
WATER TYPE
Cool
PRIMARY VEGETATION
PRIMARY VEGETATION
Seagrasses, seaweeds, and mangroves
Seagrasses, maerl, and kelp
LOCATION In the southwest of the Yucatán Peninsula, Campeche State, Mexico
Between South Uist, Eriskay, and Barra, Outer Hebrides, Scotland, UK
LOCATION
Strong tidal currents flow through the Sound of Barra, and its clear, shallow waters and sandy sea floor provide an ideal habitat for the eelgrass Zostera marina. The eelgrass beds, together with beds of maerl (see p.245), are home to many species of small animals. Such rich, current-swept communities in this part of Scotland are threatened by the building of rock causeways across the sounds, which cut off the nutrient-
bearing currents that are essential for healthy growth. Eelgrass also grows in nearby brackish lagoons, together with the tasselweed Ruppia maritima, which is regarded by some scientists as a type of seagrass. Forests of the kelp Laminaria hyperborea grow on rocks at the edges of the sound, and these are home to abundant sea squirts and sponges.
SATELLITE VIEW, WITH LAGOON AT TOP
Two channels connect this sedimentladen lagoon to the Gulf of Mexico, while three rivers feed in fresh water, producing a pronounced change in salinity. The seagrasses Thalassia testudinum, Syringodium filiforme, and Halodule wrightii cover 29 percent of the lagoon. With 448 recorded animal species, Términos is the most speciesrich of Mexico’s four large lagoons.
ATLANTIC OCEAN NORTHEAST
Falmouth Bay COASTAL TYPE
Rocky
with inlets WATER TYPE
Cool
PRIMARY VEGETATION
Laminaria hyperborea kelp, eelgrass LOCATION
Southwest Cornwall, England, UK
OCEAN ENVIRONMENTS
The coastline of Falmouth Bay includes two drowned river valleys (rias), the Fal and Helford, which are now long, sheltered sea inlets. Because of their rich marine life, these inlets,
EELGRASS STANDS
Healthy stands of eelgrass now thrive in the current-swept sound. Almost 90 per-cent of western Europe’s eelgrass was lost to a wasting disease in the 1930s.
together with part of Falmouth Bay, have been designated as a European marine Special Area of Conservation. Beds of the eelgrass Zostera marina and maerl (see p.245) in the inlets are home to a wide variety of animals, including the rare Couch’s goby. On the wave-exposed rocky coasts outside the inlets, the kelp Laminaria hyperborea grows in dense forests that support many associated seaweeds and animals.This kelp has a stiff stipe, which raises the frond off the sea bed and means that the forest has developed well vertically. On the rock beneath the kelp, there is competition for space among anemones, sponges, and smaller seaweeds, while other animals hide in kelp holdfasts. The kelp stipes have a rough surface and provide effective attachment points for red seaweeds, bryozoans, soft corals, and other types of encrusting animals. On the fronds, tiny blue-rayed limpets graze, and colorful sea slugs eat small hydroids and lacy bryozoans. In deeper water, the kelp Laminaria ochroleuca grows, close to its northern limit in Europe. This kelp is similar to Laminaria hyperborea, but has a smooth stipe on which little can grow. Two other kelps are found in the area, the sugar kelp (Laminaria saccharina), which has a crinkled frond, and furbelows (Saccorhiza polyschides), which has a large, hollow holdfast and grows up to 13 ft (4 m) long in just one season. CORNISH KELP FOREST
This forest of Laminaria hyperborea kelp has many different plants and animals living on the rocks beneath it, and on the kelp itself.
ATLANTIC OCEAN SOUTHEAST
Saldanha Bay Rocky and sandy bay with lagoon
SOUTH ATLANTIC KELP FOREST
On the west coast of South Africa, sea bamboo is the largest of the local kelps. It can grow as tall as 50 ft (15 m).
COAST TYPE
WATER TYPE
Cool
currents PRIMARY VEGETATION
Kelp and eelgrass LOCATION
Western Cape, South Africa
The cold Benguela Current flowing northward along the west coast of South Africa brings nutrient-rich water that is ideal for kelp growth, and sea bamboo (Ecklonia maxima)
is abundant in Saldhanha Bay. The smaller split-fan kelp (Laminaria pallida) becomes dominant in deeper water. South Africa is famous for its diversity of limpets, and the kelp limpet Cymbula compressa is found only on sea bamboo. Its shell fits neatly around the stipe, where it grazes. The highly endangered limpet Siphonaria compressa occurs only in the bay’s Langebaan Lagoon, grazing on the endemic eelgrass Zostera capensis.
SEAGRASS BEDS AND KELP FORESTS
149
INDIAN OCEAN WEST
Gazi Bay Shallow bay and fringing reef
COASTAL TYPE
WATER TYPE
Tropical
PRIMARY VEGETATION
Seagrasses and mangroves LOCATION
30 miles (50 km) south of Mombasa, Kenya
Gazi Bay’s shallow, subtidal mud and sand flats are sheltered by fringing coral reefs. Twelve species of seagrass grow on the mudflats, and these seagrass beds cover about half of the bay’s 6 square miles (15 square km). Mangrove-lined creeks flow into the bay, and this unusual proximity of mangrove, seagrass, and coral reef systems has led to scientific studies on how they interact. The seagrass beds proved to be important in trapping particles washed into the bay from the creeks. Most were trapped within 11/4 miles (2 km) of the mangroves. The seagrass beds provide food directly for shrimp larvae, zooplankton, shrimp, and oysters, and they are the main feeding grounds of all the fish in the bay, making them very important to the health of the local fisheries.
INDIAN OCEAN EAST
Lombok COASTAL TYPE
Semi-sheltered bays on rocky coast WATER TYPE
Tropical
PRIMARY VEGETATION
Seagrasses LOCATION
Lesser Sunda Islands, Indonesia
MANGROVES AND SEAGRASS
Unusually for mangrove-lined creeks, the water here is clear, and stands of seagrasses are able to flourish in the waterways leading into Gazi Bay.
At least 11,600 square miles (30,000 square km) of sea bed around Indonesia is covered by seagrasses. In the warm, shallow lagoons and bays, 12 species of seagrass flourish. Gerupuk Bay in the south of the island of Lombok contains 11 of the 12 Indonesian seagrass species, with Enhalus acoroides and Thalassodendron ciliatum forming dense stands. Analyses of the gut contents of fish that live among seagrass in Lombok’s waters
revealed that crustaceans were the dominant food source. However, a species of Tozeuma shrimp found there avoids the attention of predators by having an elongated body colored green with small white spots, a perfect camouflage against seagrass leaves. At low tide, local people use sharp iron stakes to dig for intertidal organisms, and this damages the seagrass leaves and roots, thereby threatening the survival of the beds.
HUMAN IMPACT
THREAT FROM TOURISM The islands of Southeast Asia contain the greatest diversity of seagrasses in the world, but human activity threatens them in many places. Tourism is a means of bringing a much-needed boost to many local economies and this necessitates the building of hotels and other tourist facilities in previously unspoiled areas.
Future tourist development in the region may threaten the Lombok seagrass beds as a result of pollution and loss of habitat through the building of beach facilities, such as marinas.
BAY OF PLENTY
The seagrass beds in Lombok’s bays are a source of seaweeds, sea urchins, sea cucumbers, mollusks, octopus, and milkfish for the area’s inhabitants.
OCEAN ENVIRONMENTS
HOTEL DEVELOPMENT
150
SHALLOW SEAS PACIFIC OCEAN WEST
Sea of Japan/East Sea COASTAL TYPE
Mainly rocky WATER TYPE
Warm to cold PRIMARY VEGETATION
Kelp and seagrasses Off the west coast of the island of Hokkaido, northern Japan LOCATION
The Sea of Japan/East Sea is influenced by the warm Tsushima Current from the south and the cold Liman Current from the north, so its marine flora is a rich mix of temperate and cold-water species because of the wide range of water temperatures in different parts of the coast. The mixing of these currents also provides plentiful nutrients for plant growth. Seagrass diversity is moderate, but eelgrasses are particularly well represented with
seven species, several of them endemic to the area. Kelps are also diverse, with species of Undaria, Laminaria, and Agarum thriving in the colder waters in the north. Kelp is highly nutritious, and Hokkaido is the traditional center of kelp harvesting. INVASIVE KELP
Since 1981, Asian kelp (Undaria pinnatifida) has spread from its indigenous sites in Japan, China, and Korea to four continents.
FEMALE RED PIGFISH IN KELP FOREST
PACIFIC OCEAN SOUTHWEST
Poor Knights Islands COASTAL TYPE
Offshore
islands WATER TYPE
Temperate
PRIMARY VEGETATION
Kelp and other brown seaweeds Off the east coast of Northland, North Island, New Zealand
LOCATION
In 1981, a marine reserve was set up around the Poor Knights Islands, extending 2,600 ft (800 m) out from the shore. The area is popular with divers for its caves and kelp forests. In the most exposed places, the kelp Lessonia variegata is predominant, while at more sheltered sites Ecklonia radiata is more abundant, together with the large brown seaweed Carpophyllum flexuosum. Large numbers of sea urchins dominate in some places.
PACIFIC OCEAN NORTHEAST
Izembek Lagoon Rocky coast and lagoon
COASTAL TYPE
WATER TYPE
Cold; low
salinity PRIMARY VEGETATION
Eelgrass and kelp On the northern side of the Alaskan Peninsula, Alaska, US
LOCATION
SEAGRASS BANKS
OCEAN ENVIRONMENTS
INDIAN OCEAN EAST
Shark Bay has one of the world’s largest seagrass beds, covering about 1,500 square miles (4,000 square km).
Shark Bay COASTAL TYPE Shallow, semi-enclosed bay
LOCATION
food for one of the world’s largest populations of dugongs (see p.419), which are preyed on by sharks. The adjacent Hamelin Pool is too salty for seagrasses, but it is well known for the growth of stromatolites (see p.232).
Shark Bay is a UNESCO World Heritage Site, and it contains one of the largest, most diverse seagrass beds in the world. Its 12 species of seagrass, which include Amphibolis antarctica and Posidonia australis, dominate the subtidal zone to depths of about 40 ft (12 m). The vast seagrass beds provide
POSIDONIA AUSTRALIS
WATER TYPE
Tropical;
high salinity PRIMARY VEGETATION
Seagrasses Inlet of the Indian Ocean, north of Perth, Western Australia
Izembek Lagoon covers 150 square miles (388 square km) of the Izembek State Game Refuge and is the site of one of the world’s largest eelgrass beds. The eelgrass Zostera marina grows in dense beds here, both subtidally and on intertidal flats, where it is grazed by wading birds at low tide. Over half a million geese, ducks, and shorebirds
stop over at the lagoon during migration to refuel on the eelgrass. On the rocky, open coasts outside the lagoon, kelp forests thrive in the cold water. The most common forestforming kelp here is bull kelp (Nereocystis luetkeana), which can grow to 130 ft (40 m) in length. Bull kelp is an annual, which means that it reaches maturity within a single year. It grows quickly, at a rate of up to 5 in (13 cm) per day. The huge fronds, which have many long, strap-shaped blades, are supported by gas-filled bladders (pneumatocysts) that are up to 6 in (15 cm) in diameter. FEEDING GROUNDS
After raising their young farther north, thousands of Brant geese graze on eelgrass in Izembek Lagoon in the fall before flying south to Baja California, Mexico.
151 PACIFIC OCEAN EAST
Monterey Bay Kelp Forest COASTAL TYPE
Rocky and sandy WATER TYPE
Cool to warm PRIMARY VEGETATION
Kelp LOCATION
South of San Francisco, California, US
The California coast is famous for its beds of giant kelp (Macrocystis pyrifera), the largest seaweed on the planet (see p.238). It forms dense forests just offshore, and in Monterey Bay it outcompetes bull kelp for sunlight in many places, but the latter dominates in more exposed areas. Inshore of these giant species, other smaller kelps thrive. The kelp forests provide a unique habitat. Sea otters (see p.402), which live among the kelp forests and eat sea urchins, are thought to be important in controlling the urchins, which graze on the kelp. Seagrasses of the genus Phyllospadix are also found in Monterey Bay. Unusually for seagrasses, they can attach to rock, and grow in the surf zone or in intertidal pools on rocky coasts. Each year over 140,000 tons of giant kelp are harvested in California for the extraction of alginates, which are used in the textile, food, and medical industries.
In exceptional circumstances, giant kelp can be 265 ft (80 m) long. The forests are at their thickest in late summer, and decline during the dark winter months.
OCEAN ENVIRONMENTS
SUNLIT FOREST
152
SHALLOW SEAS CORAL DIVERSITY
Coral Reefs
In this seascape off a Fijian island, groups of shoaling sea goldies hover over diverse species of coral, sponges, and other reef organisms.
CORAL REEFS ARE SOLID STRUCTURES
built from the remains of small marine organisms, principally a group of colony-forming animals called stony (or hard) corals. Reefs cover about 108,000 square miles (280,000 square km) of the world’s shallow marine areas, growing gradually as the organisms that form their living surfaces multiply, spread, and die, adding their limestone skeletons to the reef. Coral reefs are among the most complex and beautiful of Earth’s ecosystems, and are home to a fantastic variety of animals and other organisms; but they are also among the most heavily utilized and economically valuable. Today, the world’s reefs are under pressure from numerous threats to their health.
Types of Reefs Coral reefs fall into three main types: fringing reefs, barrier reefs, and atolls. The most common are fringing reefs. These occur adjacent to land, with little or no separation from the shore, and develop through upward growth of reef-forming corals on an area of continental shelf. Barrier reefs are broader and separated from land by a stretch of water, called a lagoon, that can be many miles wide and dozens of yards deep. Atolls are large, ring-shaped reefs, enclosing a central lagoon; most atolls are found well away from large landmasses, such as in the South Pacific. Parts of the reef structure in both atolls and barrier reefs often protrude above sea level as low-lying coral islands—these develop as wave action deposits coral fragments broken off from the reef itself. Two other types of reefs are patch reefs—small structures found within the lagoons of other reef types—and bank reefs, comprising various reef structures that have no obvious link to a coastline.
FRINGING REEF
BARRIER REEF
ATOLL
A fringing reef directly borders the shore of an island or large landmass, with no deep lagoon.
A barrier reef is separated from the coast by a lagoon. In this aerial view, the light blue area is the reef and the distant dark blue area is the lagoon.
An atoll is a ring of coral reefs or coral islands enclosing a central lagoon. It may be elliptical or irregular in shape.
OCEAN ENVIRONMENTS
coral grows on shoreline, forming fringing reef
island subsides when volcano has become inactive
sea level
BARRIER REEF
FRINGING REEF
lagoon volcanic island
ATOLL FORMATION
An atoll is shown here forming around a volcanic island. First, the island’s shore is colonized by corals forming a fringing reef (above). Over time, the island subsides, but coral growth continues, forming a barrier reef (above right). Finally, the island disappears, but the coral maintains growth, forming an atoll (right). Atolls can also form as a result of sea-level rise.
lagoon of shallow water
reef face ATOLL
coral continues to grow, forming barrier reef volcanic island becomes submerged central area filled by reef limestone coral continues to grow where waves bring food
CORAL REEFS
153
Reef Formation The individual animals that make up corals are called polyps. The polyps of the main group of reef-building corals, stony corals, secrete limestone, building on the substrate underneath. The polyps also form colonies that create community skeletons in a variety of shapes. An important contributor to the life of these corals is the presence within the polyps of tiny organisms called zooxanthellae, which provide much of the polyps’ nutritional needs. Other organisms that add their skeletal remains to the reef include mollusks and echinoderms. Grazing and boring organisms also contribute, by breaking coral skeletons into sand, which fills gaps in the developing reef. Algae and other encrusting organisms help bind the sand and coral fragments together. Most reefs do not grow continuously but experience spurts of growth interspersed with quieter periods, which are sometimes associated with recovery from storm damage. STONY CORAL
This group of branching hard corals is growing at a depth of about 16 ft (5 m) off the coast of eastern Indonesia. Individual stony corals can grow up to a few inches per year.
OPEN POLYPS
At the center of each polyp is an opening, the mouth, which leads to an internal gut. The tissue around the gut secretes limestone, which builds the reef.
Distribution of Reefs Stony corals can grow only in clear, sunlit, shallow water where the temperature is at least 64˚F (18˚C), and preferably 77–84˚F (25–29˚C). They grow best where the average salinity of the water is 36 ppt (parts per thousand) and there is little wave action or sedimentation from river runoff. These conditions occur only in some tropical and subtropical areas.The highest concentration of coral reefs is found in the Indo-Pacific region, which stretches from the Red Sea to the central Pacific. A smaller concentration of reefs occurs around the Caribbean Sea. In addition to warm-water reefs, awareness is growing about other corals that do not depend on sunlight, and form deep, cold-water reefs—some of them outside the tropics (see p.178). WARM-WATER REEF AREAS
The conditions needed for the growth of warm-water coral reefs are found mainly within tropical areas of the Indian, Pacific, and Atlantic oceans. The reefs are chiefly in the western parts of these oceans, where the waters are warmer than in the eastern areas.
COLD-WATER CORAL
This species, Lophelia pertusa, is one of a few of the reef-forming corals that grow in cold water, at depths to 1,650 ft (500 m).
HUMAN IMPACT
Bleaching refers to color loss in reef-building corals and occurs when the tiny organisms called zooxanthellae, which give corals their colors, are ejected from coral polyps or lose their pigment. In extreme cases, this can lead to the coral’s death.Various stresses can cause bleaching, including pollution, ocean temperature rise, and ocean acidification (see p.67). In recent decades, some mass bleaching events have affected reefs over wide areas.
OCEAN ENVIRONMENTS
CORAL BLEACHING
154
SHALLOW SEAS REEF CREST
Parts of a Reef
In front of the reef crest (the uppermost, seaward part of a reef), spurs of coral sometimes grow out into the sea, separated by grooves.
Distinct zones exist on coral reefs, each with characteristic levels of light intensity, wave action, and other parameters. Each zone’s characteristics determine the organisms that live there. The reef slope, or forereef, is the part that faces the sea. The upper parts of the reef slope are dominated by branching coral colonies and intermediate depths by massive forms. These are the areas of the reef with the greatest diversity of species. At the top of the reef slope is the reef crest. This takes the brunt of the wave action and is subject to high light levels. Shoreward of the reef crest is the reef flat, a shallow, relatively flat expanse of limestone, sand, and coral fragments that may become exposed at high tide. The number of corals decreases toward the shore. Barrier reefs and atolls have a final zone, the lagoon area.
sea urchin
crinoid
elkhorn coral staghorn coral maze coral
Species Diversity In addition to reef-building corals, the warm, sunny waters of a reef are populated by a huge variety of other animals as well as seaweeds. The richest and healthiest reefs are home to thousands of species of fish and other marine vertebrates, such as turtles, while all the major groups of invertebrate animals are also represented. These include sponges, worms, anemones, and non-reef-building corals (such as sea fans), crustaceans, mollusks (which include snails, clams, and octopuses), and echinoderms (sea urchins and relatives). Every nook and cranny of a reef is used by some animal as a hiding place and shelter. All the organisms in the reef are part of a complex web of relationships. Many organisms are also involved in mutualistic partnerships with other organisms, in which both species benefit. tube sponge
sea fan
star coral
QUEEN ANGEL FISH
OCEAN ENVIRONMENTS
One of hundreds of fish species found on the Caribbean reefs, this juvenile angelfish feeds on small crustaceans and algae.
lettuce coral
REEF ZONES
The structure of a typical fringing reef, including forereef, reef crest, and reef flat, and some of the sea life that inhabits it, are shown here. The forereef has three zones, which are dominated by different coral forms: branching coral, massive coral, and platy coral. Individual corals are not shown to scale.
platelike star coral finger coral sea whip
TUBE SPONGES
Different species of sponges are found in many parts of the reef, including caves and cavities, as well as on the open reef slope.
PLATY CORAL ZONE Corals in this deep, dark part of the forereef expand horizontally to capture maximum sunlight, forming platelike colonies.
CORAL REEFS
155
The Importance of Reefs beach small brain coral
SEA URCHIN
Sea urchins graze on algae and are important in preventing algal overgrowth on coral reefs.
seagrass
golf ball coral
sea anemone
Coral reefs are of inestimable value for many reasons. First, they provide a protective barrier around islands and coasts: without the reefs, these would erode away into the ocean. Second, reefs are highly productive, creating more living biomass than any other marine ecosystem and providing an important food source for many coastal peoples. Third, they support more species per square unit area than any other marine environment. In addition to known coral-reef species, scientists estimate that there may be several million undiscovered species of organisms living in and around coral reefs. This biodiversity may be vital in finding new medicines for the 21st century—many reef organisms contain biochemically potent substances that are being studied as possible cures for arthritis, cancer, and other diseases. Finally, because of their outstanding beauty, reefs contribute to local economies through tourism, particularly attracting snorkelers and scuba-diving enthusiasts (see p.474).
REEF FISHING
Small-scale fishing using hand nets, often transported to a suitable site by canoe, is common throughout the Indian and Pacific oceans, as shown here off Pantar Island in eastern Indonesia.
SAND AND ALGAL ZONE This area is dominated by sand and seagrass, which may harbor small marine life.
REEF FLAT The animals living here must be able to endure high temperatures and salinity.
REEF CREST The corals inhabiting this zone are invariably robust, as they must withstand energetic wave action.
HUMAN IMPACT
CORAL POISONING
GOLDEN CRINOID
Crinoids, or feather stars, are related to starfish. They usually live in a hole or other shelter on the reef, extending their elegant arms to catch food.
crinoid arm
Vulnerable Reefs BRANCHING CORAL ZONE This zone is just below the reef crest and is dominated by corals with branching forms, such as staghorn coral.
SUBMARINE STUDY
Here researchers record the frequency of algal species on a reef in the Hawaiian Islands, using a camera, a frame for delineating areas of reef, and underwater writing implements.
OCEAN ENVIRONMENTS
MASSIVE CORAL ZONE This central part of the forereef is usually dominated by massive corals—that is, colonies with rounded shapes.
Many types of stress can damage reefs and are doing so on a massive scale. Much of the harm is caused by human activity, including coastal pollution, uncontrolled development of coasts, and diving tourism. Other problems include collection of corals and reef organisms for the aquarium and jewelry trades, uncontrolled mining of reefs for building materials, and destructive fishing practices. Natural disturbances include tropical storms and mass die-offs of animals that help to maintain reef health. Coral bleaching, linked to rises in sea temperatures (see p.153), is particularly worrisome. Coral reefs can recover from periodic natural traumas, but if they are subjected to multiple sustained stresses, they perish. It has recently been estimated that two-thirds of the world’s warm-water reefs are at risk of disappearing in the near future.
One of the most destructive fishing practices, liable to kill corals over wide areas of reef, involves the use of poison to help catch tropical fish for the aquarium trade. This is practiced in parts of Southeast Asia such as the Philippines. The young boy photographed below, swimming at a depth of about 70 ft (20 m), carries a catch bag, net, and a squirt bottle containing a solution of sodium cyanide. The cyanide is used to immobilize selected reef fish, making them easier to catch, but kills all the living corals that it comes in contact with, taking a terrible toll on the health of the reef.
156
SHALLOW SEAS ATLANTIC OCEAN WEST
Bermuda Platform Atoll with fringing and patch reefs TYPE
150 square miles (370 square km)
AREA
CONDITION Some coral bleaching reported LOCATION Northwest Atlantic, extending west and north of the islands of Bermuda
The Bermuda Platform is the elliptical, flattened summit of a huge volcanic submarine mountain (seamount) in the northwest Atlantic. Its surface lies 13–60 ft (4–18 m) below sea level and is covered in a thick layer of limestone, formed over millions of years from the
ATLANTIC OCEAN WEST
Florida Reef Tract TYPE
Barrier reef, patch
reefs 400 square miles (1,000 square km)
AREA
BOILER REEFS
remains of corals and other organisms growing on the platform. Along the platform’s southern and eastern edges, limestone sand has gradually built up to form the Bermuda islands. Coral reefs are present around the other edges of the platform, forming an atoll, while patch reefs grow on its central surface. The diversity of reef flora and fauna here is less than that associated with the reefs in the Caribbean Sea to the south. over the past 35 years, mainly due to human impact. Live coral cover has decreased, coral diseases have become extensive, inhabitants that were once common (such as the queen conch) have virtually disappeared, and the area of reef encroached on by mats
LOCATION From east of Soldier Key, Biscayne Bay, to south of the Marquesas Keys, Florida, USA
This system of coral reefs is 160 miles (260 km) long and curves to the east and south of the Florida Keys. Some geologists classify it as a barrier reef, others as a barrier-like collection of bank reefs. It is the largest area of coral reefs in the US and has a high biodiversity, being home to more than 40 species of stony coral, 500 species of fish, and hundreds of mollusk species. The reefs’ health has declined
Bahama Banks TYPE Fringing reefs, patch reefs, barrier reef
1,200 square miles (3,150 square km)
AREA
CONDITION Localized areas of damage
OCEAN ENVIRONMENTS
LOCATION Bahamas, southeast of Florida, US, and northeast of Cuba
Lighthouse Reef TYPE
Atoll with patch
reefs
CONDITION Degraded; some recent recovery
ATLANTIC OCEAN WEST
ATLANTIC OCEAN WEST
These small reefs, close to the surface, are called “boilers” after their frothy appearance when waves break on them.
120 square miles (300 square km)
AREA
CONDITION
Western Caribbean, 60 miles (80 km) east of central Belize
LOCATION
Nevertheless, 21 different species of stony coral, 17 species of soft (non-reef-building) coral, including many spectacular purple sea fans, and about 120 different species of fish have been recorded here. of algae has expanded. Causes of this degradation include overfishing, fertilizer runoff from south Florida, increase in ocean temperature and sea level, and sewage pollution from boats. Other contributing factors include hurricane damage, declines in algae-grazing sea urchins, and direct damage from dive-boat anchors and ship groundings. Steps are being taken to reverse the decline, with some signs of success. CARYSFORT REEF
Carysfort Reef, part of the Florida Reef Tract, lies close to Key Largo and is the site of many ancient shipwrecks.
The Bahamas is an archipelago of some 700 islands scattered over two limestone platforms, the Little Bahama and Great Bahama Banks, in the West Indies. The platforms have been accumulating for at least 70 million years—the Great Bahama Bank is over 15,000 ft (4,500 m) thick—yet their surfaces remain 33–80 ft (10–25 m) below sea level. Many of the islands have fringing coral reefs; there are also many patch reefs on the Banks and a
Generally
healthy
barrier reef near the island of Andros. The reefs are home to a range of corals and coral reef-dwelling animals that is typical for the western tropical Atlantic. Although local declines in coral cover and occasional outbreaks of coral disease have been recorded, the reefs are generally healthy. There has been concern about overgrowth of algae, but for now the algae are being kept in check by a thriving population of parrotfish, which graze the reefs.
HARD AND SOFT CORALS
This diverse group of corals, including a large purple sea fan, was photographed off the island of New Providence.
Lighthouse Reef is an atoll lying 35 miles (55 km) east of the huge Belize barrier reef, off the coast of central Belize. It is roughly ovalshaped, about 23 miles (38 km) long, and 5 miles (8 km) wide on average.
CORAL REEFS Like all atolls, it is bounded by a ringlike outer structure of coral formations, many of which break the surface.These form a natural barrier against the sea and surround a lagoon, which sits on top of a mass of limestone.The lagoon is relatively deep but contains numerous patch reefs along with six small, sandy, low-lying islands, or cays (one containing a dive center). At its center is Lighthouse Reef ’s most remarkable feature—a large, almost circular sinkhole in the limestone, known as the Great Blue Hole. Approximately 410 ft (125 m) deep, this feature formed some 18,000 years ago during the last ice age, when much of Lighthouse Reef was above sea level. At that time, freshwater erosion
produced a complex of air-filled caves and tunnels in the limestone. At some point, the ceiling of one of the caves collapsed, producing what is now the entrance to the Blue Hole. Later, as sea level rose, the cave complex flooded, and it is now accessible only by adventurous scuba divers. Elsewhere, the atoll boasts large areas of mostly healthy reef, although some were affected by coral bleaching in 2010. As well as patch reefs within the atoll, around its margins are many coral-encrusted walls (dropoffs) that descend to depths of several hundred yards. Lighthouse Reef exhibits a biological diversity typical of the region; it is home to some 200 fish species and 60 species of stony corals.
157
HUMAN IMPACT
DIVING THE GREAT BLUE HOLE The Great Blue Hole is one of the world’s most exciting dive sites. It is not recommended for the fainthearted (as sharks are commonly encountered) or for novice divers (because perfect buoyancy control is needed). At 125 ft (38 m) depth, an array of impressive ancient stalactites can be seen hanging from the slanting walls of the hole. The entrance to a system of caves and tunnels lies a few yards farther down.
The water in this sinkhole extends to a depth of 410 ft (125 m), producing the deep blue color after which it is named.
OCEAN ENVIRONMENTS
GREAT BLUE HOLE
158
SHALLOW SEAS INDIAN OCEAN NORTHWEST
Red Sea Reefs TYPE Fringing, patch, and barrier reefs; atolls
6,300 square miles (16,500 square km)
AREA
CONDITION Localized areas of severe damage LOCATION Red Sea coasts of Egypt, Israel, Jordan, Saudi Arabia, Sudan, Eritrea, and Yemen
The Red Sea contains arguably the richest, most biologically diverse, and most spectacular coral reefs outside Southeast Asia. The coral reefs in the northern and southern areas of the sea differ considerably. In much of the northern section, the coasts shelve extremely steeply and there are few offshore islands. The coral reefs here are mainly narrow fringing reefs, with reef flats typically only a few yards wide, and slopes that plunge steeply toward the sea floor. In the south, off Eritrea and southwestern Saudi Arabia, is a much wider area of shallow continental shelf. Many of the reefs
INDIAN OCEAN NORTHWEST
Aldabra Atoll TYPE
Atoll
60 square miles (155 square km)
AREA
Excellent, although it has suffered some coral bleaching CONDITION
LOCATION Western extremity of the Republic of Seychelles archipelago, northwest of Madagascar
OCEAN ENVIRONMENTS
At 20 miles (34 km) long and 9 miles (14.5 km) wide, Aldabra is the largest raised coral atoll in the world. The term “raised” refers to the fact that the
in this area surround offshore islands, and there are fewer steep dropoffs. The southern Red Sea also receives a continuous inflow of water from the Gulf of Aden to its south that is high in nutrients and plankton, making the waters more cloudy, which restricts reef development. Live coral cover throughout the Red Sea reefs is generally high, at about 60–70 percent, as is the diversity of stony and soft corals, fish (including the famous Red Sea lionfish), and other reef organisms. More than 260 species of stony coral have been identified in the central Red Sea. Although many reef areas are healthy, others have been damaged by intensive diving tourism and deposition of untreated sewage. In 2010, a major coral bleaching event, linked to raised water temperatures, affected the central Red Sea. Coral predation by the crown-ofthorns starfish has also been a problem. GULF OF AQABA REEF
Groups of little red fish of the genus Anthias fluttering around hard coral heads, or colonies, are a familiar sight on Red Sea reefs.
limestone structures forming its rim, which originated from coral reefs, have grown into four islands that protrude as much as 27 ft (8 m) above sea level. Situated on top of an ancient volcanic pinnacle, the islands enclose a shallow lagoon, which partially empties and then fills again twice a day with the tides. Because of its remote location, and its status as a Special Nature Reserve and (since 1982) UNESCO World Heritage Site, Aldabra has escaped the worst of the stresses that human activities have placed on most of the world’s coral reefs. Although, in common with many Indian Ocean locations, the atoll was affected by
a severe coral bleaching event in 1997–98, its external reefs are in a near-pristine state. They are rich in marine life, featuring large schools of reef fish, green and hawksbill turtles, forests of yellow, pink, and purple sea-fans, groupers, hammerhead sharks, and barracuda. The atoll’s inner lagoon contains numerous healthy patch reefs, is fringed by mangrove swamps, and is inhabited by turtles, parrotfish, and eagle rays. On land, Aldabra is famous for its giant tortoises, rare exotic birds such as the flightless rail, and giant robber crabs, which have claws big enough to crack open coconuts.
INDIAN OCEAN WEST
Bazaruto Archipelago Fringing reefs, patch reefs
TYPE
60 square miles (150 square km)
AREA
Generally good; some damage
CONDITION
Southeastern coast of Mozambique, northeast of Maputo
LOCATION
The Bazaruto Archipelago is a chain of sparsely populated islands on the coast of Mozambique, formed where sand was deposited over hundreds of thousands of years by the Limpopo River. A Marine National Park, established in 2001, covers most of the archipelago, protecting its impressive fringing reefs and kaleidoscopic range of marine life. More than 2,000 fish species, 100 species of stony corals, and 27 dazzling soft-coral species, including unusual “green tree” corals, are found on Bazaruto’s reefs, as well as eagle rays, manta rays, and five species of turtles. The archipelago is also a refuge for one of the remaining populations of dugongs (see p.419) in the western Indian Ocean. REEF SAFARI
A peaceful way of visiting the shallow, crystal-clear waters around the Bazaruto reefs is on a dhow, as part of a reef safari.
MUSHROOM ROCK
Strong tidal flows of ocean water into and out of Aldabra’s lagoon have sculpted some raised clumps of old reef into mushroom-shaped islets known as champignons.
159 INDIAN OCEAN CENTRAL
Diego Garcia Atoll TYPE
INDIAN OCEAN CENTRAL
The numerous ringlike structures in this aerial view are faros—mini-atolls within a larger Maldivian atoll.
Maldives
Atoll
TYPE
Atolls, fringing
reefs
17 square miles (44 square km)
AREA
AREA 3,500 square miles (9,000 square km)
Generally good; recovered from coral bleaching in 1998
CONDITION
CONDITION Recovering from coral bleaching
LOCATION
Chagos Archipelago, central Indian Ocean, southwest of Sri Lanka
LOCATION
This atoll, best known as a US military base, is also home to one of the world’s largest populations of breeding sea birds. The reefs around the atoll’s edges and within its central lagoon are home to 220 species of stony coral. In 2010, the Chagos Archipelago, of which Diego Garcia forms a part, was declared a “no-take” marine reserve, making it the largest marine protected area in the world.
The Maldives are a group of 26 atolls, many of them very large, in the Indian Ocean. The majority are composed of numerous separate reefs and coralline islets (some 1,200 in all), arranged in ringlike structures. Within most of the atoll lagoons, which are 60–180 ft (18–55 m) in depth, there are usually many patch reefs and numerous structures called faros, which are rare outside the Maldives. These look like mini-atolls and consist of roughly elliptical reefs with a central lagoon. Most of the Maldivian atolls are themselves arranged in a large, elliptical ring, some 500 miles (800 km) long and 60 miles (100 km) wide. The reefs that fringe all the Maldivian atolls, islets, and faros contain more than 200 species of colorful stony coral, more than 1,000 different fish species, and are abundant in other marine life. Groupers, snappers, and sharks, for example, are frequently encountered. In 1998, a severe coral bleaching event killed up to 90 percent of the corals in some areas, and had a strong negative impact on diving tourism. Since 1998, a recovery has occurred, although another severe bleaching event took place in 2010.
WESTERN SIDE OF DIEGO GARCIA
ATOLLS WITHIN ATOLLS
Off southern India, southwest of Sri Lanka, in the Indian Ocean
HUMAN IMPACT
ATOLL CITY Male, the Maldives’ capital city, covers the entire surface area of a coral island that forms part of an atoll rim. Its reef has been mined to provide building materials for artificially extending the island.
INDIAN OCEAN NORTHEAST
Andaman Sea Reefs TYPE
Fringing reefs
2,000 square miles (5,000 square km)
AREA
CONDITION Some areas poor due to coral bleaching, diver damage
Andaman Sea coasts: Thailand, Myanmar, Andaman and Nicobar Islands, Malaysia, Sumatra
LOCATION
SOFT CORAL COLONIES
These soft corals and glass fish, which are almost transparent, were photographed off southwest Thailand.
and breeding grounds for endangered sea turtles. A coral bleaching event in 1998 badly damaged reefs around the Andaman and Nicobar islands, and in 2010 another severe widespread bleaching event affected the reefs along the coast of Thailand. The 2004 Indian Ocean tsunami caused relatively little damage. Other threats to these reefs include collection of marine life for aquariums, destructive fishing techniques, siltation caused by poorly managed deforestation on some of the islands, and anchor damage from dive boats.
OCEAN ENVIRONMENTS
Most Andaman Sea reefs are fringing reefs around islands off the coasts of Thailand and Myanmar or, in the northwest, off the eastern coasts of the Andaman and Nicobar islands—the site of the largest continuous area of reefs in south Asia. About 200 coral species and more than 500 fish species have been recorded here. The reefs and islands are also important feeding
The partly dismantled reef leaves the island poorly protected from storms, so a sea wall has been built around much of its perimeter, preventing major damage during the 2004 Indian Ocean tsunami.
160
SHALLOW SEAS PACIFIC OCEAN WEST
Shiraho Reef TYPE
Fringing reef
10 square km (4 square miles)
AREA
CONDITION Reasonable; damaged in parts by bleaching in 1998, 2007 LOCATION Southeast coast of Ishigaki Island, at the southwestern extremity of Japanese archipelago
Shiraho Reef, off Ishigaki Island, part of the Japanese archipelago, came to notice in the 1980s as an outstanding example of biodiversity, with some 120 species of coral and 300 fish
species concentrated in a few square kilometres. The reef also contains the world’s largest colony of rare Blue Ridge Coral (Heliopora coerulea). For decades, environmentalists battled to save the reef from the building of a new airport for Ishigaki. A proposal to construct the airport on top of the reef was dropped, but concern remains now that it has been built on land, as discharge of excavated soil into the reef is likely to have an adverse effect. BLUE RIDGE CORAL
Despite its name, the colour of this coral varies from violet through blue, turquoise, and green to yellow-brown. Its branching vertical plates can form massive colonies.
PACIFIC OCEAN WEST
Tubbataha Reefs TYPE
Atolls
330 square km (130 square miles)
AREA
CONDITION Good; recovering from coral bleaching in 2010
Central Sulu Sea, between the Philippines and northern Borneo
LOCATION
The Tubbataha Reefs lie around two atolls in the centre of the Sulu Sea and are famous for the many large pelagic (open ocean) marine animals attracted to them – such as sharks, Manta Rays, turtles, and barracuda. The steeply shelving reefs here are also rich in smaller life, including many species of crustaceans, colourful nudibranchs (sea slugs), and more than 350 species of stony and soft coral. In the early 1990s, the Tubbataha Reefs were rated by scuba divers among the top ten dive sites in the world. However, during the 1980s they suffered considerable damage from destructive fishing practices and the establishment of a seaweed farm.
In 1988, the Philippines government intervened, declaring the area a National Marine Park, and since 1993 it has also been a UNESCO World Heritage Site. The condition of the Tubbataha reefs has much improved, due to the enforcement of measures such as a prohibition on fishing and a ban on boats anchoring on the reefs (visiting craft must use mooring buoys). A setback occurred in January 2013 when a US Navy minesweeper ran aground on the reef, damaging over 2,000 square m (21,500 square ft).
CORAL DROP-OFF
In this photograph of a steeply shelving reef slope, several species of soft coral are visible, together with a shoal of Longfin Bannerfish.
PACIFIC OCEAN WEST
Nusa Tenggara TYPE Fringing reefs, barrier reefs
5,000 square km (2,000 square miles)
AREA
CONDITION Damaged by fishing practices
OCEAN ENVIRONMENTS
LOCATION Southern Indonesia, from Lombok in the west to Timor in the east
REEF FLAT OFF PANTAR ISLAND
This shallow reef area, featuring numerous species of stony coral and a starfish, is in east Nusa Tenggara.
Nusa Tenggara is a chain of around 500 coral-fringed islands in southern Indonesia. The northern islands are volcanic in origin, while the southern islands consist mainly of uplifted coral limestone. Many of the reefs have been only rarely explored. However, what surveys have been carried out
indicate an extremely high diversity of marine life in this region. For example, a single large reef can contain more than 1,200 species of fish (more than in all the seas in Europe combined), and 500 different species of reefbuilding coral. Common animals here include Eagle Rays, Manta Rays,
Humphead Parrotfish, and various species of octopuses and nudibranchs (sea slugs). Major threats to the reefs in Nusa Tenggara include pollution from land-based sources, removal of fish for the aquarium trade, and reef destruction by blast fishing. Coral bleaching affected some reefs in 2010.
161 PACIFIC OCEAN SOUTHWEST
Great Barrier Reef TYPE
Barrier reef
37,000 square km (14,300 square miles)
AREA
Damaged by tropical storms, pollution, and an unbalanced ecosystem
CONDITION
Parallel to Queensland coast, northeastern Australia
LOCATION
Australia’s Great Barrier Reef, which stretches 2,010km (1,250 miles), is the world’s largest coral reef system. Often described as the largest structure ever made by living organisms, it in fact consists of some 3,000 individual reefs and small coral islands. Its outer edge ranges from 30 to 250km (18 to 155 miles) from the mainland, and its biological diversity is high. The reef contains about 350 species of stony coral and many of soft coral. Its 1,500 species of fish range from 45 species of butterflyfish, to several shark species, including silvertip, hammerhead, and whale sharks. The reef is also home to 500 species of algae, 20 species of sea snake, and 4,000 species of mollusc.
PACIFIC OCEAN SOUTHWEST
Marshall Islands TYPE
Atolls
6,200 square km (2,400 square miles)
AREA
Generally good; some episodes of coral bleaching
CONDITION
Micronesia, southwest of Hawaii, western Pacific
LOCATION
REEF CHANNEL
In this view of a central area of the reef, a deep, meandering channel separates two reef platforms. The region’s high tidal range drives strong currents through such channels.
However, a study published in 2012 reported that the reef has lost more than half its coral cover since 1985. The factors causing this damage include pollution, tropical cyclones, raised water temperature causing mass coral bleaching, population outbreaks of the Crown-of-thorns Starfish, overfishing, and shipping accidents. The Marshall Islands consist of 29 coral atolls and five small islands in the western Pacific. The atolls lie on top of ancient volcanic peaks that are thought to have erupted from the ocean floor 50-60 million years ago. They include Kwajalein, the largest atoll in the Pacific at 2,500 square km (1,000 square miles), and Bikini and Enewetak atolls, which were used by the USA for testing nuclear weapons between 1946 and 1962. Human pressures on these two remote, evacuated atolls have been minimal during the past 50 years, and marine life around them now thrives; for example, 250 species of coral and up to 1,000 species of fish have been recorded at Bikini. MAJURO ATOLL
As with many Pacific atolls, the rim of Majuro Atoll consists partly of shallow submerged reef and partly of small, low-lying islands.
Hawaiian Archipelago Fringing reefs, atolls, submerged reefs
TYPE
1,180 square km (450 square miles)
AREA
Coral disease outbreaks reported
CONDITION
LOCATION
North-central Pacific
The Hawaiian Archipelago consists of the exposed peaks of a huge undersea mountain range. These mountains have formed over tens of millions of years as the Pacific Plate moves
northwest over a hotspot in the Earth’s mantle. Coral reefs fringe some coastal areas of the younger, substantial islands at the southeastern end of the chain, such as Oahu and Molokai. To the northwest, located on the submerged summits of older, sunken islands, are several near-atolls (such as the French Frigate Shoals) and atolls (such as Midway Atoll). These reefs are highly isolated from all other coral reefs in the world, and although their overall biological diversity is relatively low, many new species have evolved on them. The more remote reefs are healthy, but in 2013, a serious coral disease was reported affecting reefs on Oahu and Kauai.
One of the tiniest residents of the Great Barrier Reef, at just 7–8mm (less than 1⁄3in) long from snout to tail, is the Stout Infantfish. When discovered in 2004, the Infantfish was declared to be the world’s smallest vertebrate species. That title has since been claimed first by a slightly smaller species of Indonesian cyprinid fish, and more recently by a tiny species of frog, about 7mm (1⁄4in) long, found in Papua New Guinea.
PACIFIC OCEAN SOUTHWEST
Society Islands TYPE Fringing reefs, barrier reefs, atolls
recorded. The reefs’ health is generally good, but some reefs around the busy holiday destination islands of Tahiti, Moorea, and Bora-Bora have been severely affected by construction, sewage, and sediment run-off.
1,500 square km (600 square miles)
AREA
CONDITION Good, but significant local damage
French Polynesia, northeast of New Zealand, south-central Pacific
LOCATION
The Society Islands comprise a chain of volcanic and coral islands in the South Pacific, including islands with barrier reefs (such as Rai’atea), islands with both fringing and barrier reefs (such as Tahiti), and atolls or nearatolls (such as Maupihaa and Maupiti). The reefs’ biological diversity is moderate compared with the reefs of Southeast Asia, although more than 160 coral species, 800 species of reef fish, 1,000 species of mollusc, and 30 species of echinoderm have been
MOOREA
A wide fringing reef almost completely surrounds the shoreline of mountainous Moorea, part of which is visible in this view.
FRENCH FRIGATE SHOALS
Reef fish, including Longfin Bannerfish, Milletseed Butterflyfish, and Bluestripe Snappers, swim around a table coral.
OCEAN ENVIRONMENTS
PACIFIC OCEAN CENTRAL
THE WORLD’S SMALLEST VERTEBRATE?
THE GREAT BARRIER REEF
The warm, clear waters of the reef support an astonishing variety of life. Here, fairy basslets can be seen shoaling over vividly colored soft corals. Bright coloration can serve several purposes for reef fish, including helping members of a species to recognize each other and acting as a warning to predators.
164
SHALLOW SEAS
The Pelagic Zone THE PELAGIC ZONE IS THE WATER COLUMN ABOVE
LION’S-MANE JELLYFISH
This daunting giant of the plankton can grow up to 6 ft (2 m) across, with 200-ft (60-m) tentacles.
the continental shelf (although the term is also used to refer to the water column of the open ocean). It is a vast environment, and temperature and salinity variations within it result in distinct water masses. These are separated by “fronts” and characterized locally by different plankton. Coastal and shelf waters are more productive than the open ocean. When calm, the water stratifies, cutting off the surface plankton from essential nutrients in the layers below. Storms cause the layers to mix, stimulating phytoplankton blooms. High latitudes have seasonal plankton cycles; in warmer waters, seasonal upwelling of nutrient-rich deeper water triggers phytoplankton growth.
Microscopic Productivity Much of the primary productivity in the world’s oceans and seas occurs over the continental shelves. Tiny phytoplankton floating in the surface waters harness the Sun’s energy through photosynthesis to produce living cells. Some of the tiniest algae (picoplankton) are thought to supply a considerable amount of primary production. As well as sunlight, nutrients and trace metals are needed for phytoplankton growth. These are PRIMARY PRODUCTION often in short supply in the open This satellite map shows variations in ocean, but shelf waters benefit from a primary production, indicated by the continual input from rivers, mixing by concentration of the pigment waves and, on some coasts, the chlorophyll a in the oceans and the amount of vegetation on land. upswelling of nutrient-rich water.
DRIFTING ZOOPLANKTON
OCEAN ENVIRONMENTS
Continental shelf zooplankton contains many larvae of sea bed animals that then drift away to new areas.
CHLOROPHYLL A CONCENTRATION LOW
HIGH
VEGETATION INDEX MIN
MAX
The Plankton Cycle
Riding the Currents
In temperate and polar seas, optimal phytoplankton growth occurs in both spring and summer. There are long daylight hours and maximum nutrient levels after winter storms have mixed the water column and resuspended dissolved nutrients from the seabed. The well-known spring blooms can rapidly turn clear seawater into pea soup, or a variety of other colors, depending on the organism. Typically, there is a succession of phytoplankton species with short blooms. Responding to abundant food and increasing temperatures, tiny zooplankton begin grazing the phytoplankton and reproducing. Bottom-living coastal animals release clouds of larvae to feed in the nutritious broth, before taking up life on the sea bed. Spawning fish also contribute a mass of eggs and larvae. Eventually, the phytoplankton is grazed down, nutrients are exhausted, and productivity drops off, in an annual cycle that will be renewed again next spring.
From tiny algae to giant jellyfish, the animals and plants of the plankton either float passively or swim weakly. This is mainly to keep them up in the sunlit surface waters, where most production occurs; these drifters must go wherever the currents take them. On most continental shelves, there is a residual drift in a particular direction, although wind-driven surface currents, where most of the plankton live, can move in any direction for short periods. Some animals go on long migrations to spawn, relying on residual currents to bring their larvae back to areas suitable for their growth into adults; for example, conger eel larvae take around two years to drift back from their spawning grounds far off the continental shelf. The larvae of the majority of coastal animals, including those of barnacles, mussels, hydroids, and echinoderms, spend much shorter periods in the plankton—just long enough to disperse to new areas of coast. However, the plankton is a dangerous place, full of hungry mouths and tentacles, and though millions of eggs and larvae are released, the vast majority of planktonic feeders will die; only a lucky few find a suitable place to settle and grow.
HUMAN IMPACT
RED TIDE A rapid increase in a population of marine algae is called a bloom. This bloom on the Scottish coast, known as a red tide, was caused by the dinoflagellate Noctiluca scintillans. Sometimes blooms poison marine life. Often, the sheer numbers of organisms clog fish gills, suffocating them. Dense blooms occur naturally, but man-made pollution from nutrient runoff into the sea may also feed these blooms, making them more frequent and extensive.
COLONIZED ROPE
This rope was colonized over the course of a year by sea squirts, feather stars, fan worms, and anemones, their planktonic larvae having been transported by ocean currents.
165
MIGRATORY SHOALS
Pelagic fish such as these mackerel move around the ocean in response to temperature changes. They are among the pelagic zone’s larger predators.
Pelagic Fisheries
The animals of the plankton, especially small crustaceans such as copepods and krill, are eaten by fish, mainly small, shoaling species such as herring, sand eels, sardines, and anchovies. Most of these fish live permanently in midwater, using the seabed only to spawn or to avoid predators. They are strong swimmers (nekton), using speed to catch prey and evade predators. They can travel long distances against residual currents to feed and also to reach their spawning grounds. Small, shoaling fish are, in turn, food for larger predators, such as squid, tuna, cetaceans, NEKTONIC INVERTEBRATE and sharks. Whale sharks, basking Squid are the only invertebrates sharks, and baleen whales are among that swim strongly enough to be the largest of the marine animals, yet classed as nekton. They catch a variety of prey including fish and they feed directly on plankton, planktonic crustaceans. consuming vast quantities.
Continental shelf waters support massive quantities of pelagic fish, ultimately sustained by abundant plankton. The most important fisheries are for herring, sardines, anchovies, pilchards, mackerel, capelin and jackfish. Squid are also fished commercially. Many fish stocks are under severe pressure as boats and nets get bigger and the technology to pinpoint shoals becomes ever more sophisticated. Pelagic fish and squid are caught in drift nets that hang about 30 ft (10 m) down from the surface. In the north Pacific, some 105,000 miles (170,000 km) of drift net is available to major fisheries; unfortunately, these nets also trap cetaceans, turtles, and diving birds. Drifting longlines are used for tuna and swordfish; these also catch juvenile fish, sharks, turtles, and seabirds. Midwater trawls capture vast quantities of shoaling fish such as herring, mackerel, and sardines. Small-scale fisheries for a wide variety of other pelagic species are important in sustaining local coastal communities worldwide.
FOOD CHAIN THREAT
Sand eels are food for sea-birds (such as this Arctic tern), seals, cetaceans, and larger fish. Despite their importance at the base of many food chains, vast quantities are taken by fisheries for feeding to livestock and farmed fish, and are burned as fuel oil.
OCEAN ENVIRONMENTS
Active Swimmers
THE STEEL-BLUE WAVES of the open ocean
conceal an extraordinary landscape, where the continents plunge down to a vast, undulating, muddy plain. Here, the ocean water column supports layer upon layer of life, from the surface zone, powered by sunlight, to the crushing pressure of the darkest depths. In places, the abyssal plain is broken by underwater volcanoes or by mountain ranges high enough to rival the Himalayas. Springs of super-hot water emerge from these mountainsides, supporting living communities unlike any others on the planet. Elsewhere, Earth’s vast tectonic plates collide, ripping trenches in the ocean floor and stimulating powerful earthquakes. Yet fewer people have explored these mysterious depths than have flown in space.
TH E OP E N O C E A N A N D OC E A N F L O O R BENEATH THE WAVES
In the deepest ocean, an underwater Mount Everest could be hidden beneath these waves—and still leave plenty of space to put the Burj Khalifa (the world’s tallest building) on top. As a result, we have better maps of the Moon than of the deep seabed.
168
THE OPEN OCEAN AND OCEAN FLOOR
OCEAN ZONES
SUNLIT ZONE 0–660 ft
Zones of the Open Ocean CONDITIONS IN THE OCEAN
vary greatly with depth. Light and temperature changes occur quickly, while pressure increases incrementally. Although many of these changes are continuous, the ocean can be divided into a series of distinct depth zones, each of which produces very different conditions for living things.
The Surface Layer The top three feet of the ocean is the richest in nutrients. This upper layer is sometimes called the neuston, although this term is also used for the animals that live there, such as jellyfish. Amino acids, fatty acids, and proteins excreted by plants and animals float up into this surface layer, as do oils from the decomposing bodies of dead animals. These produce a rich supply of nutrients for phytoplankton. The top three feet of seawater is also the interface where gas exchange takes place between the ocean and the atmosphere. This is vitally important to all life on Earth, as half of the oxygen animals need for survival comes from the ocean. Not surprisingly, phytoplankton gathers in this surface zone in daylight, as do the animals that feed on them. This zone is also highly susceptible to chemical pollution and floating litter, which can be deadly for marine life. NOCTURNAL AND DIURNAL DISTRIBUTION
Only a small proportion of marine life inhabits the deep zone; the majority live above 3,300 ft (1,000 m). The sunlit zone is dangerous for animals—many stay in the twilight zone by day and only go upward at night. The sunlit zone is much emptier by day. NIGHT
OCEAN ENVIRONMENTS
DAY 10% sunlit zone
40% sunlit zone
75% twilight zone
50% twilight zone
15% deep zone
10% deep zone
HUMAN IMPACT
FREE DIVING When divers breathe compressed air underwater, excess nitrogen dissolves in their blood, and they risk the bends if they surface too fast. Free divers avoid this by holding their breath underwater. Pressure squeezes their lungs, but the surrounding blood vessels swell to protect them, and blood nitrogen levels stay safe. Trained free divers can hold their breath long enough to reach 660 ft (200 m), using aids to help them descend and ascend.
Seawater rapidly absorbs sunlight, so only one percent of light reaches 660 ft (200 m) below the surface. Phytoplankton use the light to photosynthesize, forming the base of food chains. This zone drives all ocean life.
TWILIGHT ZONE 660–3,300 ft Too dark for photosynthesis, but with just enough light to hunt by, many animals move from this zone into the sunlit zone at night.
DARK ZONE 3,300 ft–13,100 ft Almost no light penetrates below 3,300 ft (1,000 m). From here to the greatest depths, it is dark, so no plants can grow, and virtually the only source of food is the “snow” of waste from above. Temperatures down here are a universally chilly 35–39oF (2–4oC), and the pressures so extreme that only highly adapted animals can survive. The dark zone is defined as continuing down to the abyssal plain, below 13,100 ft (4,000 m). Technically, all the water below 3,300 ft (1,000 m) is a dark zone, where the only light comes from bioluminescent animals (see p.224). However, for convenience, the waters below the dark zone can be further subdivided.
ABYSSAL ZONE 13,100–19,700 ft Beyond the continental slope, the sea bed flattens out. In many areas, it forms vast plains at depths below 13,100 ft (4,000 m). Some areas drop deeper to a sea floor that undulates down to depths of 19,700 ft (6,000 m). Around 30 percent of the total seabed area lies between these depths. Animals living here move up and down through a narrow column above the sea bed, called the abyssal zone.
HADAL ZONE 19,700–36,100 ft The sea floor plunges below 19,700 ft (6,000 m) in only a few deep ocean trenches. This hadal zone makes up less than 2 percent of the total seafloor area. Fewer than 10 human beings have ever visited this zone (see p.183), and the pressures are so high that only a few submersibles are able to operate here. As yet, little is known about life at these depths, although anemones and jellyfish have been observed at a depth of 27,000 ft (8,221 m) and a fish has been dredged from a depth of 27,500 ft (8,370 m). Amphipods, as well as amoebae and various other microbes, have been found living at the bottom of the Mariana Trench, the deepest point in the oceans.
Ligh
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ZONES OF THE OPEN OCEAN
DEEPEST OCEAN POINTS
ne Mari
The column below shows the average depth (yellow band) and greatest depth (red band) of the oceans and some of the world’s seas.
Life lphin
o m) d (201 9 ft ) 5 6 m • (300 48 ft • 1,1g penguin in k m) (680 hark s 30 ft • 2,2 at white ) m 0 gre 0 ,0 t (1 e l 00 f • 3,3 rm wha m) 00 spe t (1,2 turtle 37 f • 3,9therback ) 0m lea (1,58 3 ft seal 8 1 , t • 5 phan ele
The Sunlit Zone
North Sea average depth 308 ft (94 m) Baltic Sea greatest depth 1,473 ft (449 m) North Sea greatest depth 2,296 ft (700 m) Arctic Ocean average depth 3,248 ft (990 m) Mediterranean Sea average depth 4,921 ft (1,500 m) Caribbean Sea average depth 4,960 ft (1,512 m)
almost one-third of the total sea-bed area is made up of abyssal plains at around 14,800 ft (4,500 m).
Atlantic Ocean average depth 10,925 ft (3,330 m)
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CRYSTAL WATERS
Crystal-clear tropical waters look idyllic, but the clarity indicates that there are few nutrients and therefore few phytoplankton in the water. As a result, feeding for animals is quite poor.
The sunlit zone is the range in which there is enough sunlight for photosynthesis. The ocean absorbs different wavelengths of sunlight to differing extents (see p.36). Nearly all red light is absorbed within 30 ft (10 m), so red animals look black below this depth. Green light penetrates much deeper in clear water, down to around 330 ft (100 m), and blue light to twice that. Due to the presence of chlorophyll, phytoplankton preferentially absorb the red and blue portions of the light spectrum (for photosynthesis) and reflect green light. They can photosynthesize down to about 660 ft (200 m) in clear water. In cloudy water, the sunlit zone is shallower, because light is absorbed more quickly. The accumulations of phyto- and zooplankton in fertile waters absorb sunlight, reducing the depth of the sunlight zone. Phytoplankton must stay in the sunlit layer during daylight to photosynthesize. Zooplankton follows them there to feed, along with animals that feed on zooplankton. This zone is dangerous because light makes animals conspicuous to their hunters.
Indian Ocean average depth 12,762 ft (3,890 m) Pacific Ocean average depth 14,041 ft (4,280 m) Southern Ocean average depth 14,763 ft (4,500 m)
Mediterranean Sea greatest depth 16,715 ft (5,095 m) (Hellenic Trough) Arctic Ocean greatest depth 18,377 ft (5,601 m) (Molloy Deep)
Southern Ocean greatest depth 23,466 ft (7,152 m)
FEEDING IN THE SUNLIT ZONE
The phytoplankton of the sunlit zone is the food of zooplankton. Larger animals, such as these shrimp, in turn feed on zooplankton. Phytoplankton is at the bottom of most ocean food chains.
Caribbean Sea greatest depth 25,213 ft (7,685 m) (Caymen Trench) Indian Ocean greatest depth 25,344 ft (7,725 m) (Java Trench)
Atlantic Ocean greatest depth 29,404 ft (8,962 m) (Puerto Rico Trench)
Pacific Ocean greatest depth 35,829 ft (10,920 m) (Mariana Trench)
ZONES OF LIFE
The different zones of life in the deep ocean are shown here, together with the depths reached by humans and a selection of marine animals. Most life is concentrated above 3,300 ft (1,000 m), where there is some light.
Phytoplankton must remain in the sunlit zone if it is to catch enough sunlight for photosynthesis. This zone is the warmest and richest in the nutrients needed for growth. It would be counterproductive to expend huge amounts of energy to stay in this zone, so phytoplankton has developed a wide range of mechanisms to help it hang there effortlessly. Buoyancy bubbles, droplets of oil, or stores of light fats keep some species afloat. Others are covered in spines, which increase their surface area and help buoy them up. Some phytoplankton forms colonial chains, which produce more drag in water and slow the rate at which the phytoplankton sink. One group, called dinoflagellates, have threadlike flagellae that let them swim weakly. In this highly productive zone, phytoplankton produces half of the oxygen in the atmosphere. In temperate regions, phytoplankton proliferates in summer, sometimes forming dense blooms.
DIATOM
Diatoms are a very prolific type of phytoplankton. Some grow colonially, attached to rocks in chains or mats. Each year, six billion tons of phytoplankton grow in the oceans worldwide.
ZOOPLANKTON
This sample of zooplankton, collected in a net, includes an echinoderm (bottom left), a radiolarian, and a crab larva (center), with a fish egg (bottom right).
OCEAN ENVIRONMENTS
Beyond the abyssal plains, undulating, rocky seabed stretches down to around 19,700 ft (6,000 m). Only the ocean trenches reach deeper.
Living in the Sunlit Zone
170
THE OPEN OCEAN AND OCEAN FLOOR
Plankton and Nekton
The Twilight Zone
In spring, as phytoplankton blooms begin to develop, zooplankton start multiplying. They follow the phytoplankton into the sunlit zone to feed. Most are herbivores that feed on phytoplankton; some are carnivores that hunt other zooplankton. Many are classed as meroplankton—the young of animals like crabs, lobsters, barnacles, and some fish—which have a planktonic larval stage and use the currents to spread. By taking advantage of the summer phytoplankton feast, they avoid competing for food with adults of their own kind. While plankton drift with the currents, many free-swimming animals (collectively called nekton) gather to feed on them: fish, squid, marine mammals, and turtles. These, in turn, are food for predatory fish and seabirds. Some larger animals, such as basking sharks, also feed on zooplankton and nekton. SARGASSUMFISH
Here, two Sargassumfish are hiding in Sargussum seaweed, floating on the surface of the Sargasso Sea.
GIANT FILTER-FEEDER
OCEAN ENVIRONMENTS
More than 36 ft (11 m) long, basking sharks like this one scoop up shoals of plankton, then filter them from the water with the white gill rakers inside their jaws.
In the twilight zone, there is just enough light for animals to see—and be seen. As a result, predators and prey are in constant battle. Many species are almost totally translucent, to avoid casting even a faint shadow. Others are reflective, to disguise themselves against the light from above, or have wafer-thin bodies that reduce their silhouette. To cope with dim light, many animals in this zone have large eyes. COPEPOD The main source of food here is detritus. Copepods are herbivores. They Many animals therefore migrate upward into make up 70 percent of the total the sunlit zone, where food is plentiful, at zooplankton population, with thousands in a cubic yard. night, returning to the twilight zone as the Sun rises. Millions of tonnes of animals, equivalent to around 30 percent of the total marine biomass, make this daily trek—by far the largest migration of life on Earth. The length of the journey is a matter of scale. Small planktonic animals measuring less than 1mm (1/25 in) in length may only migrate through 20 m (70 ft), but some larger shrimp travel 600 m (2,000 ft) each way, every day.
ZONES OF THE OPEN OCEAN
The Dark and Abyssal zones
SQUID OF THE OPEN OCEAN
Squid live in several zones of the ocean, from the sunlit zone, where this big-fin reef squid is found, to the deep zone. Deep-sea squid are difficult to photograph and are often photographed only as dead specimens.
The waters below the twilight zone are all dark, cold, subject to high pressure, and impoverished in food. For animals adapted to these deep zones, pressure is not a problem: their liquid-filled bodies are almost incompressible, compared to the gasfilled bodies of surface-living birds and mammals, which are much more easily compressible and subject to the effects of pressure. Most fish use gases in their swim bladders to maintain buoyancy, and these are susceptible to pressure change. Many deep-water fish therefore have no functional swim bladders. For most deep-water species, lack of food is the biggest problem: only about five percent of the energy that plants produce at the surface filters down to these depths. Animals of the deep are typically slow-moving, slow-growing, and long-lived. They conserve energy by waiting for food to come to them. Many therefore have massive mouths and powerful teeth. Others use tricks to catch prey: anglerfish dangle lures, and some species even harbor luminescent bacteria or use chemical processes to make these lures and other structures glow.
SPOOKFISH
mucus on body attract bacteria, which protect it from heat
DISCOVERY
THE CHALLENGER Many oceanographic discoveries were made by HMS Challenger, a converted British warship that made a 68,900mile (110,900 km-) voyage around the oceans from 1872 to 1876, collecting depth soundings as it went. In March 1875, near Guam, it dropped a sounding line to a depth of 26,850 ft (8,184 m) and collected clay to prove this was the seabed. By good luck, the ship was over the Mariana Trench, close to the deepest spot in the ocean, now appropriately called the Challenger Deep.
171
The brownsnout spookfish is found at depths of up to 3,300 ft (1,000 m), on the boundary of the dark zone. Its bones are so thin that it is almost transparent, and its large eyes look upward to spot predators attacking from above. It feeds mainly on copepods, and gives birth to live young that float in the plankton.
red tentacles around head gather food and provide sensory information
HEAT-TOLERANT WORM
This polychaete worm (a type of segmented worm) was discovered by the Alvin submersible in 1979— and named Alvinella pompejana in its honor. It is the most heat-tolerant animal on Earth, living near water emerging from hydrothermal vents at 570˚F (300˚C).
The Hadal Zone
DEEP SQUEEZE
ALVIN SUBMERSIBLE
Alvin is designed to withstand the extreme pressure of the deep zone and has enabled scientists to make many important discoveries during over 4,000 dives.
FANGTOOTH FISH
The fangtooth has been recorded at depths of 16,380 ft (4,992 m). Like many deep-water fish, it has a large head and massive teeth. Sensory organs along its body detect prey movement in the dark.
OCEAN ENVIRONMENTS
A polystyrene cup attached to the outside of a submersible resurfaces at a fraction of its original size, illustrating the effects of pressure in the deep ocean.
Few deep-water species have been observed in their natural environment of the hadal zone and even fewer photographed. Many species are known only from samples dredged up in nets, and most photographs are of dead specimens (including the fangtooth on the right). Sometimes deep-sea animals can be studied in aquariums, but many species cannot survive temperature and pressure changes when brought to the surface. Although many of the animals here hunt each other, the food chain must begin with a supply of food from above. Whereas animals on the seabed can patrol large areas to find food particles accumulated there over weeks and months, animals in midwater must grab food particles in the short time when they float downward past them, which is much trickier. Only a small proportion of the detritus from above is harvested in midwater, so food is always scarce. Scientists observing this zone often see the same species repeatedly. The environment of this zone is remarkably uniform worldwide and there are few physical or ecological barriers to block the movement of species. Many deep-water species therefore are widely distributed, and several are found in every ocean. As a result, species diversity is low: only around 1,000 of the known 29,000 fish species live at this depth.
GLOBAL EXPLORER
The Global Explorer is an ROV that can dive to 10,000 ft (3,045 m). Controlled by the mother ship through a cable, it can take photographs of the sea floor.
173
Exploration with Submersibles Submersibles are underwater vehicles, smaller than
scuba diver C-Quester Deepflight Super Falcon nuclear submarine White shark Pisces class DSV gulper eel Hercules ROV
3,300 ft (1,000 m)
6,500 ft (2,000 m) 9,800 ft (3,000 m) 13,100 ft (4,000 m)
16,400 ft (5,000 m) shinkai
19,700 ft (6,000 m)
jiaolong
23,000 ft (7,000 m) 26,300 ft (8,000 m) 29,500 ft (9,000 m)
Deepsea Challenger Nereus ROV
32,800 ft (10,000 m) 36,100 ft (11,000 m)
SHALLOW EXPLORATION
C-QUESTER Developed by Netherlandsbased company U-boat Worx, C-Quester submersibles allow one or two people to explore down to 330 ft (100 m).
PISCES IV An example of a Deep Submergence Vehicle (DSV) used in scientific research, Pisces IV is owned and operated by the Hawaii Undersea Research Laboratory. It carries three people and can operate down to 6,600 ft (2,000 m).
SHINKAI 6500 Launched in 1989 by the Japan Marine Science and Technology Center, Shinkai 6500 is one of the deeper-diving DSVs. In June 2013, it transmitted the world’s first live broadcast from 16,500 ft (5,000 m).
HERCULES ROV A fairly typical ROV, Hercules can descend to a depth of 13,500 ft (4,000 m) and take high-definition images. It is equipped with six thrusters that allow it to “fly” in any direction, like a helicopter. Slightly positively buoyant, it will gently float up to the surface if its thrusters stop turning.
DEEPSEA CHALLENGER This 24-ft- (7.3-m)- long submersible reached the Challenger Deep in March 2012, carrying the film director James Cameron. In doing so, it won what had been called the “race to inner space”—the first solo manned mission to reach the deepest spot in the oceans.
OCEAN ENVIRONMENTS
Alvin
Sea Level
MULTI-PERSON DEEPWATER RESEARCH
Recreational vehicles, such as the Super Falcon, generally descend to depths of no more than 660 ft (200 m). Most DSVs and ROVs have maximum depths varying from 3,300 ft (1,000 m) to 23,000 ft (7,000 m), but a few can go to the deepest spot in the oceans, the Challenger Deep of the Mariana Trench at 36,100 ft (11,000 m). As of early 2014, only two DSVs (including Deepsea Challenger) and two ROVs —Kaiko (Japan) and Nereus (US) have ever achieved this feat.
DEEPFLIGHT SUPER FALCON The latest of several submersibles designed by American engineer Graham Hawkes, the Super Falcon is an underwater vehicle intended mainly for private recreational exploration. It “flies” through the water, carrying two people down to 400 ft (120 m).
REMOTELY OPERATED VEHICLES (ROVS)
Into the Deep
TYPES OF SUBMERSIBLES
RACE TO INNER SPACE
submarines, used mainly for exploration, scientific study of the oceans, and recreation. First developed in the 1960s, they have helped open up the deep ocean to exploration. Modern submersibles include manned vehicles of various types and Remotely Operated Vehicles (ROVs), which are unmanned Remotely Operated Vehicles. Some recent designs no longer depend on ballast and buoyancy tanks to control descent and ascent, instead using technologies originally developed for flight. A famous manned submersible is Alvin, operated by the Woods Hole Oceanographic Institute (US). In 1977, Alvin’s crew discovered the first hydrothermal vents (see pp. 188-89) and, in 1986, it was involved in exploring the wreckage of the Titanic. During 20112013, Alvin underwent a complete rebuild. Along with Shinkai 6500 (Japan), Jiaolong (China), and others, it belongs to a class known as Deep Submergence Vehicles or DSVs. These are mostly used for scientific research. Other submersibles include ROVs, which are usually connected to a surface vessel by a tether, and those used mainly for shallow water recreation. The more sophisticated ROVs can drill cores in the sea floor and take sonar surveys, as well as record images.
174
THE OPEN OCEAN AND OCEAN FLOOR
Seamounts and Guyots SEAMOUNTS ARE TOTALLY
submerged, undersea mountains that rise at least 3,300 ft (1,000 m) from the sea floor; smaller ones are called sea knolls. Guyots are seamounts that once rose above sea level—as a result, they have a flat top caused by erosion. Often isolated in deep ocean, seamounts and guyots provide a habitat for marine life adapted to shallower water. The obstruction of a seamount forces nutrient-rich, deep-sea currents to rise closer to the surface, forming eddies above the seamount. These trap nutrients and support plankton, which in turn attract shoals of fish.
Geological Origins
HENRY GUYOT Arnold Henry Guyot (1807–1884) was the first professor of geology at Princeton University. He set up a system of weather observatories that led to the formation of the US Weather Bureau. Guyots were named in his honor by a later Princeton geology professor, Harry Hass. Hass discovered guyots using echo-sounding equipment during World War II.
EVOLUTION OF A GUYOT
Seamounts start as undersea volcanoes, where a rift in the sea bed allows volcanic eruptions. Many arise at rifts on the crest of mid-ocean ridges, formed by the movement of tectonic plates (see p. 185). Because these rifts are generally linear, seamounts tend to be elliptical or elongated in shape. They are made of volcanic basalt rock, but a thin layer of marine sediment accumulates over time. Seamounts often occur in chains or elongated groups, either because there are several weak spots along a rift, or because a series of seamounts originated sequentially at a single, stationary volcanic hotspot. Sometimes volcanic eruptions break above the ocean surface to form island chains, and these may continue out to sea as a line of guyots, or tablemounts. Newly formed volcanic rock is easily eroded, so over time, the above-water peak of the volcanic island is eroded down to a flat top. Then, as the ocean plates carry it away from the zone of SEAMOUNT FORMATION volcanic activity, the A seamount forms from an flat-topped guyot sinks underwater volcanic eruption. beneath the surface. Erosion here is slower than on
direction of plate movement
A
B
A guyot (A) begins life when a volcano erupts above a “hotspot,” creating a small volcanic island.
1
C
A
Over millennia, erosion reduces the island to a flat top at sea level, while it (A) moves away from the hotspot. A new island (B) forms.
2
B
As the island moves farther, it sinks and forms a guyot. New islands (B and C) erupt from the hotspot.
3
Upwellings
land, so it remains conical.
World Distribution
OCEAN ENVIRONMENTS
PEOPLE
There may be 100,000 seamounts and guyots in the oceans, but few have been mapped or explored and the total number is unknown. Seamounts may occur either singly or in clusters or chains, reflecting zones of past volcanic activity. The Pacific, with its Ring of Fire, is the most volcanically active ocean, containing over 30,000 seamounts and guyots. Pacific chains typically form in a northwesterly direction, matching the direction of plate movement, with 10 to 100 seamounts in each chain, sometimes connected by an undersea ridge. In the Atlantic and Indian oceans, by contrast, seamounts mostly occur singly. DISTRIBUTION MAP OF SEAMOUNTS
Some seamounts and guyots arise over volcanic hotspots, often in chains. Others form singly along mid-ocean ridges. Total numbers are unknown.
The open ocean is mainly barren, because cold, nutrient-rich currents are confined to deep water, far beneath the reach of plankton. Seamounts—which stand up to 13,000 ft (4,000 m) above the sea bed— form a major obstruction to these currents, diverting them and pushing them upward. This brings an upwelling of nutrients into the sunlit zone, and allows phytoplankton to flourish. As these nutrient-rich currents rush over the top of the seamount, they split in two and sweep around it. This makes the water above the seamount rotate, encircling a cylindrical column of still water that trapped nutrients spiral and plankton extends high above the height flow still of the seamount. This “virtual” water cylinder is called a Taylor Column. Above a seamount, it forms an area of backeddies and still water in which nutrients accumulate and plankton get trapped. upwelling This creates a zone of incredible richness and seamount productivity above the seamount—an “oasis” flow splits in the nutrient desert of the open ocean. deep-water current
WATER COLUMNS
The currents spiraling around and over a seamount create a column of still water above it. Plankton thrive on the nutrients trapped there.
A
SEAMOUNTS AND GUYOTS MOUNTAINS IN THE SEA 8,850 ft (2,700 m) 9,000 ft (2,750 m) 9,200 ft (2,800 m) 9,350 ft (2,850 m)
chain of large seamounts
This false-color map shows how a chain of seamounts has arisen, close to where two spreading tectonic plates have been displaced sideways by a transform fault. Other seamounts occur singly away from the ridge.
175
N
isolated seamount
transform fault isolated guyot
crest of East Pacific Rise
9,500 ft (2,900 m) 9,700 ft (2,950 m) 9,850 ft (3,000 m) 10,000 ft (3,050 m) 10,200 ft (3,100 m) 10,300 ft (3,150 m) 10,500 ft (3,200 m) 10,700 ft (3,250 m) 10,800 ft (3,300 m) 11,000 ft (3,350 m) 11,150 ft (3,400 m) 11,300 ft (3,450 m) 11,500 ft (3,500 m) 11,650 ft (3,550 m) 11,800 ft (3,600 m) 12,000 ft (3,650 m) 12,150 ft (3,700 m)
East Pacific Rise
12,300 ft (3,750 m)
fracture zone
12,500 ft (3,800 m) HUMAN IMPACT
ROUGHY TROUBLE
DEPTH
Life on a Seamount
PRIMNOID CORAL THREAT
Scientists fear some primnoid coral species may be wiped out by bottom-trawl fishing before they have even been named.
FEEDING FROM THE CURRENTS
This squat lobster, or pinch bug, is a scavenger living on rock faces. Currents welling over the Bowie Seamount in the northeast Pacific supply rich pickings.
SEAMOUNT FEEDER
Found in the tropics, this octocoral is a colony of soft corals. The feeding polyps, lined up along the branches, catch food from currents sweeping over the seamount.
OCEAN ENVIRONMENTS
Some seamounts were first detected when fishermen discovered large shoals of fish in the area. The nutrient-rich waters trapped above seamounts support dense concentrations of phytoplankton as well as the zooplankton that feed on them. Free-swimming animals are attracted by this feast, including fish at densities found nowhere else in the open ocean. Predators such as sharks and seals also gather to feed. Seamount rock is colonized by suspension feeders—animals that catch plankton and detritus as it floats past. Only about one in a thousand seamounts has been explored underwater. However, in studies of 25 seamounts in the Tasman and Coral seas, 850 species (some previously thought extinct) were recorded. Seamounts are important biodiversity hotspots, with up to one-third of species found there restricted to a single seamount or group of seamounts.
Fishermen thought they had found a bonanza in the 1980s when they discovered the huge shoals of fish that live over seamounts. For example, they could catch 100 tons of orange roughy (see below and p.352) in a single day. However, roughy, which can live for almost 150 years, is slow-growing, and does not produce eggs until it is 20–30 years old. Such heavy fishing cannot be sustained. World catches have declined hugely, and the roughy is now in danger.
176
THE OPEN OCEAN AND OCEAN FLOOR
The Continental Slope and Rise THE CONTINENTAL SLOPE AND RISE
CONTINENTAL MARGIN
are areas of sloping sea floor that lead from the continental shelf to the abyssal plain. Beyond a point on the shelf called the shelf break, the sea bed begins to drop more steeply. This is the continental slope, which leads into the open ocean. It sweeps down to 9,800–14,800 ft (3,000–4,500 m), where the seabed flattens out. In places, the slope is broken by submarine canyons. Sediments wash down these canyons, and accumulate at the base of the slope in a gentler gradient, forming the continental rise.
A typical continental margin is shown here, including the transition from a shoreline to the abyssal plain via the continental shelf, slope, and rise. The continental slope is about 87 miles (140 km) wide, and the continental rise is about 60 miles (100 km) wide. The vertical scale has been exaggerated: the continental slope actually has a gentle gradient, of about 1 in 50 (2 percent); and the rise is even gentler, at about 1 in 100 (1 percent).
submarine canyon shelf break— around 660 ft (200 m) below surface
slumped sediments form continental rise
Continental Slope The rock of the continental slope is blanketed by sediments washed from the land that have accumulated over millions of years. Crustaceans, echinoderms, and many other animals live in, or on, these sediments. The slope is dissected by deep canyons. These have been cut by an abrasive mix of sediment and water, called turbidity currents, which flow down the gorges at 50–60 mph (80–100 km/h). Some submarine canyons are massive: the Grand Bahama Canyon in the Caribbean has cliffs rising 14,060 ft (4,285 m) from the canyon floor. Many canyons are seaward extensions of great rivers. At the canyon end, the sediment is deposited as a spreading outwash fan, extending far out onto the abyssal plain.
outwash fan at foot of canyon
large outwash fan extending onto abyssal plain
erosion gullies
CANYON AND GULLIES
This sonar image shows a deep submarine canyon in the continental slope off Sodwana Bay, in KwaZulu Natal, South Africa.
submarine canyon
OCEAN ENVIRONMENTS
Life on the Continental Slope
CATCHING SABLEFISH
Sablefish are caught with longlines, 2/ 3 mile (1.2 km) long, that reach down toward the continental slope.
Like the shelf, the continental slope is enriched by nutrients washed off the land. This helps support both midwater (pelagic) and bottom-dwelling (demersal) fish. Fish stocks over most continental-shelf regions have declined dramatically in recent decades, as a result of overexploitation and poor management, driving more fishermen to seek deeper-water species over the continental slope. Unfortunately for fisheries, although deep-water species are SABLEFISH long-lived, they breed slowly, and stocks take a Sablefish breed slowly, it takes 14 years long time to recover. So and to replace each fish many fisheries are now caught. Fish farms (right) in serious decline. may be a better option.
ABYSSAL PLAIN This flat plain is formed by a deep accumulation of sediments. It typically lies at a depth of 15,000 ft (4,500 m).
THE CONTINENTAL SLOPE AND RISE
past shoreline formed by higher sea level in past
submarine canyon extends from shelf to abyssal plain
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some sediments deposited as delta at river mouth
sediments carried down river to sea
present shoreline
material from uplands is gradually eroded and washed into rivers
GANGES DELTA
The Ganges River carries 2 billion tons of sediment a year. Some is deposited in this massive delta. More is carried out to sea where it forms a deep-sea fan over the Bay of Bengal. TUBE ANEMONES
These sea anemones bury their bodies in sediment, at depths of 13,100 ft (4,000 m), feeding with their tentacles.
DISCOVERY SHORELINE Shorelines are shaped by erosion and deposition and move with changing sea levels.
CONTINENTAL RISE Deeper sediments build up, creating a gentle gradient of less than 1 in 100.
CONTINENTAL SLOPE The slope drops to 9,800 ft (3,000 m) at a gradient of 1 in 50.
COASTAL PLAIN An area of low-lying, flat land between the uplands and the sea.
CONTINENTAL SHELF The continental shelf is typically 460–660 ft (140–200 m) below the surface. Its width varies greatly.
Continental Rise
SEDIMENT FEEDER
Brittlestars are among the most common animals found feeding on the sediment of the continental rise. central disk
mouth (on underside of disk) five arms, arranged radially
LIZARDFISH HABITAT
The highfin lizardfish is found on the abyssal plain and continental rise, typically below about 6,600 ft (2,000 m), in water colder than 39ºF (4ºC).
STAKING A CLAIM Lured by vast oil reserves, oil companies have begun drilling in waters as deep as 7,550 ft (2,300 m) on the continental slope. These waters are also increasingly important for fisheries, so coastal countries want to establish national waters where they have sole rights to these resources. Under current maritime law, the rights of a coastal state over certain resources, such as oil, extend out to the continental margin—essentially to the boundary between continental rise and abyssal plain—or to 200 nautical miles from the coast, whichever is the greater (but never exceeding 350 nautical miles).
OCEAN ENVIRONMENTS
The continental rise is a thick wedge of sediment, up to 9 miles (15 km) deep, formed from material that has slumped downward to the base of the continental slope. This wedge drops gently away toward the abyssal plain. These sediment mounds are particularly extensive where several deep-sea fans meet and coalesce at the foot of submarine canyons. The geological boundary between the continental and oceanic crusts is completely obscured beneath these sediments. The sediments of the continental rise merge into the abyssal plains beyond. Brittlestars and polychaete worms, a type of segmented worm, live on the sediments, surviving on detritus falling from above. Atlantic red crabs scavenge on the seabed, migrating up the continental slope to breed. Deep-sea cod, Dover sole, rockfish, goosefish, and thornyheads are among the demersal species living on the slope and rise.Trawling has damaged many of these habitats, but the deeper canyons remain havens of biodiversity.
MOUNTAINS These rocks formed on an ancient sea bed and were later uplifted. Erosion will eventually return them to the sea.
COLD-WATER COMMUNITY
A squat lobster shelters among the polyps of the cold-water stony coral Lophelia pertusa, or tuft coral, in a Norwegian fjord.
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Cold-water Reefs
The map below shows the global distribution of cold-water reefs. Some of these reefs are small, while others cover up to 770 square miles (2,000 square km), although the map dots exaggerate their extent. The many reefs detected in the north Atlantic probably reflect the intensity of surveying there, particularly in the search for oil. More detailed surveys of other oceans are likely to reveal the existence of further deep-sea reefs.
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DEEP-SEA CORALS
GONIOCORELLA CORAL This deep-sea coral thicket is made mostly of Goniocorella dumosa, a species that is restricted to the Southern Hemisphere. It forms reefs at depths to 5,000 ft (1,500 m). SQUAT LOBSTER This tiny squat lobster is sitting on Madrepora oculata coral polyps, 1,290 ft (390 m) down in the Bay of Biscay, north of Spain. CHIROSTYLUS CRABS Many animals live among the coral. These long-limbed crabs are crawling over a black coral in the northeast Atlantic.
DAMAGED REEF Fishing gear has snagged on this reef west of Ireland, tearing off chunks of living reef that could be up to 8,500 years old. In 2005, the European Union banned fishing near the Darwin Mounds.
TRAWL MARK Even before scientific surveys discovered them, many deep-water reefs had been severely damaged by trawls dragged across the sea bed to catch bottom-living fish. The scarred seabed shown here is at a depth of 2,900 ft (885 m).
OCEAN ENVIRONMENTS
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LOPHELIA REEF This Lophelia reef lies deep in the Atlantic off the west coast of Ireland, where it can be studied only by means of a submersible. Fortunately, Lophelia can also be viewed in water as shallow as 128 ft (39 m) in some Norwegian fjords.
ASSOCIATED MARINE LIFE
Location of Deep-sea Reefs
LIFE IN COLD WATER
THREATS FROM DEEP-SEA TRAWLING
Deep-sea corals were first discovered in 1869, but it took the advent of sonar and deep-sea submersibles to reveal the size and abundance of the reefs that they build. Although less well studied than their tropical counterparts, these cold-water reefs are just as rich in life. The stony corals that form deep-water reefs flourish in water temperatures of 39–55ºF (4–13ºC). Unlike tropical corals, they can live in total darkness because they do not rely on zooxanthellae (p.153) living inside them to produce nourishment by photosynthesis in sunlight. Instead, they survive by filtering food from the water. Some scientists have suggested that there may be a link between the existence of deep-water reefs and the seepage of certain substances, such as methane, from the seafloor. Methane may provide energy for bacteria at the bottom of a food chain, which are then filtered from the water by the coral polyps. One of the biggest areas containing cold-water reefs—covering 38 square miles (100 square km)— was discovered during an oil-related survey of the Atlantic Frontier, northwest of Scotland, in 1998. Lophelia pertusa is the main reef-forming coral at these reefs, which are situated in an area called the Darwin Mounds and lie at a depth of 3,300 ft (1,000 m). Lophelia reefs occur at similar depths on many seamounts in the Atlantic, and also in shallow cold water such as in Norway’s fjords. Several other coral species form cold-water reefs elsewhere in the world. For example, in the Pacific, the main reef species on seamounts and oceanic banks around Tasmania and New Zealand are Goniocorella dumosa and Solenosmilia variabilis. Over 1,300 species of animals have been recorded on deep-sea reefs, and they may be important nursery grounds for commercial fish species.
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THE OPEN OCEAN AND OCEAN FLOOR
Ocean Floor Sediments OVER VAST AREAS OF THE SEABED, THE UNDERLYING
landforms are hidden beneath deep layers of sediments. Made up of silts, muds, or sands that have built up over 200 million years, they now form a blanket that is several miles thick in places. The sediments have various origins. One group, terrigenous sediments, come from land, mainly from fragments of eroded rock that are carried down rivers into the sea, then down the continental slope to form the continental rise and abyssal plain beyond. Other sediments are biogenic, formed from the hard remains of dead animals and plants. A few, called authigenic sediments, are made up of chemicals precipitated from seawater. There are even cosmogenic sediments, which come from outer space as particles in space dust and meteors. All accumulate to form extensive, flat plains. Various animals feed here and burrow into the sediments for shelter.
Deep-sea Sediments
SEDIMENT THICKNESS 0
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MAPPING SEDIMENTS
Ocean sediment depths can be measured and mapped using echosounding. Some areas (white on the map above) are still unsurveyed. Sediment is thickest near land. Glaciers also carry many sediments into the oceans.
OCEAN ENVIRONMENTS
PTEROPOD OOZE Pteropods are small winged snails that float in midwater. When they die, their internal shells of aragonite (calcium carbonate) sink to the seabed, contributing to biogenic oozes. The presence of pteropod remains in samples collected from deep in the ooze reveal changes over millennia in water temperatures and sea levels.
The average thickness of sediments on the ocean floor is 1,500 ft (450 m), but in the Atlantic Ocean and around Antarctica, sediments can be up to 3,300 ft (1,000 m) deep. Closer to the continents—along the continental rise—sediments washed from the land accumulate more rapidly, and can be up to 9 miles (15 km) deep. In the open ocean, further from the source of terrigenous sediments, the buildup rate is very slow: from a fraction of an inch to a few inches in a thousand years. That is slower than the rate at which dust builds up on furniture in an average house. The accumulated sediments tell scientists a great deal about the last 200 million years of Earth’s history. Their form and arrangement provide a vivid snapshot of sea-floor spreading, the evolving varieties of ocean life, alterations in Earth’s magnetic field, and changes in ocean currents and climate. WHITE CLIFFS OF DOVER
These chalk cliffs originated on the sea bed from a biogenic ooze, formed from algal scales (coccoliths) that built up to form layers hundreds of yards thick. They are now raised above sea level.
Sediments Derived from the Land Most terrigenous sediments come from the weathering of rock on land and are swept into the oceans, mainly by rivers but also by glaciers, ice sheets, and wind. Coastal erosion adds to these sediments. Often, they are washed down through submarine canyons to the deeper ocean. Sometimes, the route from land to sea is more indirect: volcanic eruptions eject material into the upper atmosphere before it falls as “rain” into the ocean. In the deepest ocean floors, below about 13,000 ft (4,000 m), the main sediment is red clay, composed mostly of fine-grained silts that have washed off the continents and accumulated incredibly slowly—about 1/32 in or 1 mm per thousand years. These clays may include up to 30 percent of fine, biogenic particles and have four main mineral components—chlorite, DUST STORM illite, kaolinite, and montmorillonite. RESULTS IN SILT Winds from arid regions, Clay types depend on origin and climate. such as North Africa For example, chlorite dominates in polar (shown in this satellite regions, kaolinite in the tropics, and image) carry dust far out montmorillonite is produced by to sea, where it sinks to volcanic activity. form silts.
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OOZE-FORMING ZOOPLANKTON
These radiolarians are single-celled planktonic animals. After death, their skeletons, made of silica glass, sink to the seabed, accumulating as sediments.
Feeding on the Ooze Biogenic Oozes
COCCOLITHOPHORE
When this coccolithophore dies, its platelets will add to the calcareous ooze.
SEA CUCUMBER FEEDING
Sea cucumbers wander widely over the seabed, sucking up the sediment and then extracting its organic content.
FORAMINIFERA
The tiny shells of dead foraminiferans add to the biogenic oozes.
tube feet enable animal to traverse sediment while foraging
OCEAN ENVIRONMENTS
Biogenic sediments are formed mainly from the shells and skeletons of microscopic organisms that sink to the seabed after death. The decaying remains of larger organisms, such as molluscs, corals, calcareous algae, and starfish, add to this accumulation. Oozes are calcareous if derived from the calcium carbonate shells of foraminifera, pteropods, and coccolithophores (microscopic algae), or siliceous if derived from the silica shells of single-celled radiolarians or diatoms. Because silica dissolves rapidly in seawater, siliceous oozes only build up beneath zones of high primary production. As calcareous shells and skeletons sink, they reach a depth (around 15,000 ft/ 4,500 m) where the water becomes more acidic; this, combined with pressure, means calcareous remains are dissolved rapidly in seawater at depth. Calcareous oozes therefore occur only above this “calcium carbonate compensation depth,” beneath which the seabed consists mainly of terrigenous red clays.
The “snow” of calcareous and siliceous remains from the upper levels accumulate on the ocean floor, providing the main source of food for animals living in or on the sediments. Bacteria live in the ooze, where they break down organic remains. In turn, they— along with other organic matter—are consumed by multitudes of tiny foraminiferans. Nematodes, roundworms, isopods, and small bivalve mollusks live and feed in the mud. Brittlestars feed on the ooze by sweeping food off its surface with their arms. Sea pens, crinoids, and glass sponges, which are anchored to the seabed, filter organic particles from the water column.
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THE OPEN OCEAN AND OCEAN FLOOR
Abyssal Plains, Trenches, and Mid-ocean Ridges
mouth barbel
body
OVER VAST AREAS, THE SEABED IS COVERED BY
a flat expanse of accumulated sediments. The sparse life here relies on food falling from above. In places, the abyssal plains are disrupted by more dramatic features, created by tectonic shifts. Where tectonic plates diverge, magma wells up through the gap to create mid-ocean ridges, at which new seabed is constantly being formed. At the other extreme, where plates collide, one plate is dragged downward, opening up a trench.
SEA-BED SCAVENGERS
Hagfish feed on animal corpses that fall to the abyssal plain. Blind and jawless, these primitive fish are attracted by smell. They bore into corpses, using their horny teeth, and secrete clouds of mucus to deter other scavengers.
Abyssal Plains Over large areas of the ocean floor, sediments have built up a blanket several miles thick, obscuring the underlying topography. This produces vast flat or gently undulating abyssal plains at a typical depth of 14,800 ft (4,500 m). These are most common in the Atlantic, where the Sohm Plain alone covers 350,000 square miles (900,000 square km). Abyssal plains lie at different depths, with barriers between them, and this leads to submarine waterfalls, where water spills over the barrier and down into the plain below, at rates of up to 5 mph (8 kph). Occasional abyssal storms also occur, stimulated, in a way not yet fully understood, by instabilities at the ocean surface resulting from atmospheric conditions. Originally thought to be a world without seasons, recent studies have shown that life here responds to pulses of food from above, for instance when the summer bloom of plankton dies and sinks. Most animals in this zone are scavengers with a body temperature close to that of the surrounding water. They move and grow slowly, reproduce infrequently, and live longer than their relatives at the surface.
ABYSSAL FLOOR
OCEAN ENVIRONMENTS
A recent study off the east coast of North America revealed 798 species buried in a small sediment sample from the seabed.
Earth’s outer layer of rock in continental areas is called continental crust
the steep continental slope goes down to about 10,000 ft (3,000 m)
ocean currents carve a deep gorge, called a submarine canyon, in the continental slope the continental shelf is the flooded edge of a continent, which was once dry land
silt carried down a canyon spreads out at the bottom as a fan-shaped deposit
the gently sloping continental rise is a region that extends down from the continental slope
an underwater plateau is a large, flat-topped mound caused by a few million years of underwater volcanic eruptions
MANGANESE NODULE
In places, the abyssal plain is littered with potato-sized nodules of manganese, often contaminated with other valuable metals such as nickel, cooper, and cobalt.
when a volcanic island sinks, it eventually becomes a flat-topped seamount, or “guyot”
direction of plate movement
at the mid-ocean ridge two plates pull apart and magma rises up between them, cooling then solidifying, making a new tectonic plate
each tectonic plate is made of crust and the top layer of the mantle
melted rock is called magma when it occurs beneath Earth’s surface, and lava when it is found above Earth’s surface
ABYSSAL PLAINS, TRENCHES, AND MID-OCEAN RIDGES Mariana Trench
Japan
Ocean Trenches
seamounts
China Pacific Ocean
Ocean trenches are created by a process called subduction. Where oceanic and continental tectonic plates collide, the denser but thinner oceanic plate is forced down beneath the thicker but less dense continental plate, and plunges to its destruction in the mantle deep below. Where two oceanic plates collide, the older plate is subducted beneath the younger. The buckling where the plates collide causes a deep depression at the point of impact—an ocean trench. These are the deepest places on the ocean floor. Trenches are typically V-shaped, with steeper slopes on the continental side. The Pacific is the region of most active subduction, with 17 of the 20 major ocean trench systems. The Atlantic has MARIANA TRENCH two major trenches, the Puerto Rico and South The Mariana Trench is roughly Sandwich trenches, and the Java Trench is the only 1,600 miles (2,500 km) long and 40 miles major trench in the Indian Ocean. The deepest (70 km) wide. It lies in the western trench on Earth is the Mariana Trench, located in Pacific, around 1,000 miles (1,600 km) the Pacific Ocean, near the Mariana Islands. to the south and east of Japan.
Life in the Ocean Trenches
THE DEATH OF A WHALE Occasionally, a dead whale sinks to the abyssal plain and provides a feast. Scientists have counted 12,000 animals of 43 species feeding on the bones of a single whale. It may take them 11
the abyssal plain is a flat expanse of mud that covers a vast area of seafloor
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years to strip the flesh from a blue whale. Later, bacteria invade and decompose the remaining bones. This process leaches out sulfides that sustain a complex community of seafloor life.
volcanic islands form an arc parallel to the ocean trench
each volcanic island is the above-water part of a huge undersea volcano
Animals have been found at great depths in the ocean trenches. The depth record for a fish belongs to a cuskeel, Abyssobrotula galathea. This was dredged in 1970 from a depth of 27,453 ft (8,370 m) in the Puerto Rico Trench. In 1998, the unmanned Japanese submersible Kaiko collected some large amphipods (shrimplike crustaceans) called Hirondellea gigas from the bottom of the Mariana Trench. These were later found to harbor wood-dissolving enzymes in their gut, suggesting they can digest woodfall (tree debris swept into the ocean that eventually sinks to the bottom). Kaiko also collected sediment samples that contained 432 different species of foraminiferans and a range of bacteria. Since 2010, some giant unicellular organisms more than 4 in (10 cm) across, belonging to a class called xenophyophores (a form of foraminiferan), have been observed in the Mariana Trench and elsewhere. Cameras on board the Deepsea Challenger GELATINOUS BLINDFISH that descended to the bottom A small number of these curious fish have been of the Mariana Trench in collected from the seabed in the Atlantic, 2012 detected sea cucumbers Pacific, and Indian Oceans, at depths of at least and a jellyfish as well as 10,000ft (3,000m). Like many deep-water fish, xenophyophores and amphipods. they are almost transparent, with tiny eyes. THE OCEAN FLOOR
oceanic crust is thinner than continental crust, and made of dark-colored rock
the ocean trench forms where one tectonic plate moves under another
magma pools in a chamber beneath a volcano
a volcano forms from a buildup of lava when magma erupts at the surface
DISCOVERY
THE TRIESTE EXPEDITION In 1960, two oceanographers, Don Walsh and Jacques Picard, dived to 35,797 ft (10,911 m) in the Challenger Deep section of the Mariana Trench in the bathyscaphe Trieste—still the greatest depth reached by humans. It took five hours to descend to that depth, and after just 20 minutes hanging there, the crew began their return to the surface.
OCEAN ENVIRONMENTS
The seafloor lies about 2.3 miles (3.7 km) below the sea surface. It is made of a layer of dark-colored rock, called oceanic crust, which is covered in muddy sediment. Tectonic plates are generally made of this oceanic crust and continental crust, along with part of Earth’s deeper mantle layer. Features such as volcanic islands and seamounts are caused by erupting magma.
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THE OPEN OCEAN AND OCEAN FLOOR
The Ring of Fire
SAFE HAVEN
All around the margins of the Pacific Ocean, tectonic plates are colliding. This produces a belt of intense volcanic and earthquake activity encircling the Pacific, known as the Ring of Fire. It extends for 18, 600 miles (30,000 km) in a series of arcs, from New Zealand, through Japan, and down the west coast of the Americas to Patagonia. About threequarters of the Pacific lies over a single plate, the Pacific Plate, which is colliding around its edges with the North American, Australian, and various minor plates. In the eastern Pacific, the smaller Cocos and Nazca plates are colliding with the Caribbean and South American plates. As the edges of the Pacific, Cocos, and Nazca plates subduct (move down) beneath the younger, less dense edges of neighboring plates, massive slabs of rock shatter explosively along faults, producing earthquakes. A series of deep ocean trenches, arranged in arcs around the Ring of Fire, mark the boundaries where the subducting plates move beneath neighbouring plates. Parallel to these trenches—typically at a distance of 100 miles (160 km) and always on the side of the overriding plate—are arcs of often highly active volcanoes, taking either the form of volcanic islands or (on the eastern side of the Pacific) forming lines of volcanoes on land, such as the volcanoes of Central America.
Mid-ocean-ridge islands offer protected breeding places for many sea birds, with rich feeding provided by upwelling currents offshore. The sooty tern is found in all tropical seas. It nests on oceanic islands. Ascension Island once provided safe nesting for 50,000 pairs, until humans introduced rats and cats, more than halving the sooty tern population.
THE RIDGE ON LAND
For most of its vast length, the Mid-Atlantic Ridge is hidden deep beneath the ocean. However, at Iceland, where both the Eurasian and North American plates are separating, it rises above the surface.
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OCEAN ENVIRONMENTS
Red on this map shows areas of volcanic activity around the Pacific Ocean, highlighting the Ring of Fire. These volcanoes form on continental plates as oceanic plates are thrust below.
MOUNT ST. HELENS
Mount St. Helens in Washington is part of the Ring of Fire. It erupted in May 1980, blowing the whole top off the volcano. Here, in 2004, a new lava cone has begun to grow, producing steam.
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Mid–ocean Ridges New sea bed is produced wherever tectonic plates diverge. As plates move apart, they create a rift. Magma wells up through this rift from the Earth’s mantle, forming volcanoes and creating an underwater mountain chain, called a mid-ocean ridge. The lava cools as it meets the seawater, and solidifies in vertical basalt dikes or fields of pillow lava (see p.42). Mid-ocean ridges are assembly lines along which new ocean floor is being produced. The ridges and lava fields remain visible for some time before sediments accumulate over them. Sometimes the volcanoes extend above sea level, producing islands such as Iceland. Some mid-ocean ridges spread slowly, allowing deep rift valleys to form down their centers— others are much faster-spreading but lack rift valleys. Sometimes the ridges are disrupted sideways by transform faults. As the new sea bed spreads outward, tensions are created, making it crack. Water seeps into these cracks and reemerges from hydrothermal vents (see p.188). The oceanic ridge system is the third largest feature on the Earth’s surface, after the oceans and continents.
PILLOW LAVA
Under the high pressure of the deep ocean, lava oozes slowly from the mid-ocean crests. When it meets cold seawater, it cools rapidly to form globular masses, called pillow lavas due to their shape. About 1.4 square miles (3.5 square km) of new sea floor is formed each year along mid-ocean ridges.
ASCENSION ISLAND
Ascension Island arises where the Mid-Atlantic Ridge protrudes above sea level in the south Atlantic. It covers 35 square miles (90 square km) and ascends to 2,817 ft (859 m) on Green Mountain. Sooty terns and sea turtles breed around its shores.
Ridges of the World
OCEAN WANDERERS
Macquarie Island, on the Macquarie Ridge, provides a nesting site for the black-browed albatross. Outside of the breeding season, it wanders the Southern Ocean.
The longest mid-ocean ridge occurs where the Eurasian and African plates are diverging from the North and South American plates. The Mid-Atlantic Ridge runs along this boundary for 10,000 miles (16,000 km), from the Arctic Ocean to beyond the southern tip of Africa, rising 6,000–13,000 ft (2,000–4,000 m) above the sea floor. A chain of volcanoes runs down its length, most famously in Iceland. An eruption close to Iceland in 1963 created a new volcanic island, Surtsey. Ascension Island lies very close to the ridge, and the Azores straddle it, while St. Helena and Tristan da Cunha arise from isolated volcanoes, displaced from it. A valley, 15 miles (25 km) wide, extends along the ridge crest. In the Pacific the main ridge system is the East Pacific Rise. This is Earth’s fastest-spreading system, separating at 5–6 in (13–16 cm) per year. A series of mid-ocean ridges encircle Antarctica, along the divergent boundaries between the Antarctic Plate and its neighbors, and the Carlsberg Ridge runs down the center of the Indian Ocean. Mid-Atlantic Ridge with eastern section displaced by fault in southern part of map
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Atlantis fracture zone
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THE MID-ATLANTIC RIDGE -44˚
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This section of the Mid-Atlantic Ridge has been displaced by the Atlantis Transform Fault. Transform faults occur where two plates slide sideways against each other.
OCEAN ENVIRONMENTS
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SATELLITE OCEANOGRAPHY
Sensors mounted on satellites use various wavelengths to monitor the Earth’s surface, atmosphere, and oceans, as illustrated in this computer-graphic montage of the Indian Ocean. Visible light, infrared radiation, and microwave data are all processed and projected onto maps that chart the ocean’s physical parameters. Satellites update the maps on a weekly, daily, or hourly basis to monitor ocean dynamics.
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Oceanography from Space FEATURES STUDIED FROM SPACE CLOUDS Cloud cover is detected using visible-light cameras, and cloud-top height data is derived from infrared radiometers on satellites such as Meteosat. These systems are used to track storms and forecast the weather.
RAINFALL The Tropical Rainfall Measuring Mission uses a microwave radiometer to see through clouds and detect the presence of liquid water in the atmosphere. Rainfall measures are used in computer models of the climate and ocean.
WEATHER PLANT LIFE
The world’s oceans are too vast to be adequately studied using ships alone. Even if all of the depth soundings that were taken during the 20th century were to be plotted, the resultant map would provide only sparse information on the sea floor and would even be blank in large areas. The advent of satellite remote sensing in the 1960s brought a revolution in oceanography. For the first time, it was possible to take a picture showing an entire ocean basin. Hurricane tracking and warning was one of the first benefits to accrue from early weather satellites. Eventually, a large range of sensors were developed to probe the physical attributes of the ocean surface and the atmosphere above. Ocean colour and temperature, sea-level height, and surface roughness are among the parameters that can be monitored in some detail. Satellite-derived information is a vital component of practical applications such as weather forecasting, commercial fishing, oil prospecting, and ship routing. In some cases, 30 years of continuous observations have been built up, helping scientists to track seasonal and long-term changes in the ocean environment and understand its effects on the global climate.
CHLOROPHYLL Ocean colour cameras use wavelengths of visible light to measure the concentration of chlorophyll, which is present in phytoplankton. This information is used for water-quality assessment, finding fish, and in various aspects of marine biology. MICROWAVE SCATTEROMETER Surface wind speed and direction are measured by satellites that bounce radio beams off the surface of the ocean. Windinduced ocean waves modify the return signal, and the data can be used for meteorology and climate research.
Satellites cannot directly measure the depth of the sea floor, but it can be derived from the height of the sea surface. The sea is not flat. Water piles up above gravity anomalies caused by ocean-floor features such as seamounts, producing variations in the surface that are much larger than those produced by tides, winds, and currents. By comparing the height of the sea surface against a reference height, the depth of the sea floor can be estimated.
WIND SPEED
Measuring Ocean Depth from Space
satellite orbit
ocean floor
SURFACE TEMPERATURE Infrared radiometers can measure the temperature of the sea surface precisely. Shifts in ocean currents, cold-water upwelling, and ocean fronts can be monitored for ocean and climate research.
signal is bounced from satellite off ocean surface and timed water surface varies according to sea-bed profile
reference surface
dish to track satellite height and to receive data
TEMPERATURE
the longer the signal time the lower the ocean surface
SYNTHETIC APERTURE RADAR Imaging radar systems, such as the one carried by Radarsat, penetrate clouds and can operate through the dark of the extended polar night to monitor ice shelves, sea-ice, and icebergs all year round.
THERMAL GLIDER A new generation of instrument platforms is being developed to sample the vast subsurface volume of the world’s oceans. Autonomous Underwater Vehicles, or “sea gliders”, can undertake long cruises, surfacing every day to return their data via satellite communication links.
OCEAN ENVIRONMENTS
the shorter the signal time the higher the ocean surface
satellite
ICE COVER
JASON-2 SATELLITE Jason-2 carries a radar altimeter, which is similar to the instruments on aircraft that measure their height above the surface of the Earth.
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THE OPEN OCEAN AND OCEAN FLOOR
Vents and Seeps HYDROTHERMAL VENTS ARE SIMILAR
to hot springs on land. Located near ocean ridges and rifts, at an average depth of 7,000 ft (2,100 m), they spew out mineral-rich, superheated seawater. Some have tall chimneys, formed from dissolved minerals that precipitate when the hot vent water meets cold, deepocean water. The mix of heat and chemicals supports animal communities around the vents—the first life known to exist entirely without the energy of sunlight. Elsewhere, slower, cooler emissions of chemicals called hydrocarbons occur from sites known as cold seeps.
DISCOVERY
DISCOVERING WHITE SMOKERS The first hydrothermal vents that scientists observed from Alvin, a submersible, in 1977, were black smokers. Scientists then explored other sites near mid-ocean ridges and found more vent systems. Some looked different: their fluids were white, cooler, and emerged more slowly from shorter chimneys. These were called white smokers (see right).
Hydrothermal Vents
DISTRIBUTION OF VENTS AND RIDGES
Since their discovery in 1977, hydrothermal vents have been found in the Pacific and Indian Oceans, in the mid-Atlantic, and even in the Arctic, always near mid-ocean ridges and rifts.
Hydrothermal vents always form close to mid-ocean ridges and rifts (see p.185), where new ocean crust is forming and spreading, and where magma from the Earth’s mantle lies relatively close to the surface. Seawater seeps into rock cracks opened up by the spreading sea floor. It penetrates several miles into the newly formed crust, close to the hot magma below. This heats the water to 660–750ºF (350–400ºC). The high pressure at these depths stops it from boiling, and it becomes superheated, dissolving minerals from the rocks that it is passing through, including sulfur which forms hydrogen sulfide. The hot water rises back up through cracks and erupts out of the vents as a hot, shimmering haze, complete with its load of minerals.
Black and White Smokers As superheated water erupts from a hydrothermal vent, it meets the colder water of the ocean depths. This causes hydrogen sulfide in the vent water to react with the metals dissolved in it, including iron, copper, and zinc, which then come out of solution in the form of sulfide particles. Sometimes these form pools on the seabed. However, if the water is particularly hot, it spouts up a little before being chilled by the surrounding seawater, and the metal sulfides form a cloud of black, smokelike particles. Some of these minerals form a crust around the “smoke” plumes, building up into chimneys that can reach more than 100 feet in height. Such vents are called black smokers. More recently, a different form of vent has been discovered. In these, 375°C the black sulfides come out of solution as solids well beneath 710°F the sea floor, but other minerals remain in the vent water. Silica and a white mineral called anhydrite form the “smoke” from these chimneys, which, because of their color, are called white smokers. 2°C
OCEAN ENVIRONMENTS
35°F
mineral chimney, which can grow at up to 12 in (30 cm) per day
seepage of seawater down through cracks in the oceanic crust
SMOKING CHIMNEYS
The minerals from black smokers, like this one, can increase the height of a chimney by an incredible 12 in (30 cm) a day. However, the chimneys are fragile, and they collapse when they get too high.
THE FORMATION OF A SMOKER
Water, heated by magma deep beneath the seabed, dissolves minerals from the rocks. When it erupts through vents, the water is chilled by the surrounding sea. This makes minerals precipitate as smoky clouds, which can be white or black; other minerals are deposited to form chimneys.
black cloud of metal sulfide particles
white cloud of silica and anhydrite particles
250°C 480°F shaft or conduit
unique ecosystems, including tube worms, develop in clusters around some smokers
cold water descends through cracks
heated water rises through vents
superheated water reaching temperatures above 750°F (400°C)
hot rock, or magma, heats water that has seeped into crust
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Life Without Sunlight The first biologists to explore hydrothermal vents were amazed at the life they saw. Masses of limpets, shrimps, sea anemones, and tube worms cluster close to the vents, beside unusually large clams and mussels. White crabs and a few fish, such as the eelpout, scrabble among them. Not every vent system is the same: in the Atlantic, there are no tube worms, clams, or mussels, but lots of white shrimps. Some animals that live in darkness depend on sunlit waters for their food supply but vent animals are remarkable in that they do not need sunlight for energy. White mats of bacteria around vents are the key. They oxidize sulfides from the vent water to make energy, and VENT FISH are the vent animals’ food source. This fish, called an eelpout, Some animals have the bacteria feeds on mussels, shrimps, and crabs living around vents. living inside their bodies. GHOSTLY CRAB
The hydrothermal vent crab is one of many vent creatures. Each year, about 35 new species living around vents are being described by scientists.
DIFFERENT ANIMAL COMMUNITIES
Animal communities vary between vent systems. Vents on the MidAtlantic Ridge are inhabited by swarms of rift shrimps (shown here), feeding on sulfide-fixing bacteria, but there are no giant clams.
Cold Seeps
OCEAN SMOKER
This black smoker, seen from Alvin, is similar to the one that scientists first observed in 1977, spewing out dark fluids from deep in the ocean crust.
LIFE ON A SEEP
Mussels containing methane-fixing bacteria live alongside tube worms,soft corals, crabs, and an eelpout at this cold seep, 9,800 ft (3,000 m) down on the seabed near Florida.
WORM WITHOUT A MOUTH The vent tube worm (below) can be 6 ft (2 m) long and as thick as a human arm. It has no apparent way of feeding. However, its body sac contains an organ called a trophosome, filled with grapelike clusters of bacteria. The worm’s crimson plumes collect sulfides from vent water, and the bacteria use these to produce organic material, which the worm absorbs as food.
OCEAN ENVIRONMENTS
The discovery of hydrothermal vents proved that not all deep-sea life depends on sunlight for energy. Soon, other seabed communities were found that could survive in the dark. In the Gulf of Mexico, diverse animal colonies live in shallow waters near where oil companies drill for petroleum. Here, seeps of methane and other hydrocarbons (compounds containing carbon and hydrogen) ooze up from rocks beneath the sea. Mats of bacteria feed on these cold seeps, providing energy for a food chain that includes soft corals, tube worms, crabs, and fish. Other animal communities in deep-sea trenches off the coasts of Japan and Oregon, US, rely on methane, which is released by tectonic activity. Cold-seep communities may be more common than first thought at depths below 1,800 ft (550 m), although there is often no obvious seepage. Such communities may instead rely on chemical-rich sediments exposed by undersea landslides or currents.
THE TWO POLAR OCEANS are the Arctic
Ocean in the Northern Hemisphere and the Southern Ocean, which surrounds the continent of Antarctica, in the Southern Hemisphere. They differ from other oceans in several respects, not least in the sheer quantity of ice that floats on them. This includes sea ice, which is frozen seawater, and icebergs and ice shelves, which are frozen fresh water. The polar oceans contain fewer temperature layers than other oceans, being uniformly cold, and they have different circulation patterns, which are partly wind-driven but also influenced by such factors as river inflow (in the Arctic Ocean) and sea-ice formation. The edges of the sea ice are biologically productive zones where plankton blooms occur in summer, attracting many fish, birds, and mammals.
P OL A R O C E A N S PENGUINS UNDER THE ICE
These emperor penguins are swimming in a break in the sea ice off the coast of Antarctica. They can dive to 2,000 ft (600 m), staying down for up to 20 minutes.
POLAR OCEANS
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Ice Shelves AN ICE SHELF IS A HUGE FLOATING
ice platform, formed where a glacier, or group of glaciers, extends from a continental ice sheet over the sea. The landward side of an ice shelf is fixed to the shore, where there is a continuous inflow of ice from glaciers or ice streams that flow down from the ice sheet. At its front edge, there is usually an ice cliff, from which massive chunks of ice break off (calve) periodically, forming icebergs. Ice shelves are almost entirely an Antarctic phenomenon, with only a few small ones in the Arctic.
PEOPLE
SIR JAMES CLARK ROSS The British naval officer Sir James Clark Ross (1800-1862) spent his early adulthood exploring the Arctic. In 1839, he set off to find the south magnetic pole, and on January 11, 1840 reached Antarctica, near the western side of what is now called the Ross Sea. Later, Ross and his crew discovered an ice cliff 165 ft (50 m) high. This was later named the Ross Ice Shelf.
ICE CLIFF
Antarctic Ice Shelves Fimbul Ice Shelf Lazarev Ice Shelf Ekstrom Ice Shelf
Anta
Weddell Sea
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Riiser Larsen Ice Shelf Brunt Ice Shelf
Larsen Ice Shelf
Amery Ice Shelf
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Wilkins Ice Shelf
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Filchner Ice Shelf Ronne Ice Shelf
George VI Sound Abbot Ice Shelf
West Ice Shelf
A N TA R C T I C A Shackleton Ice Shelf
Ross Ice Shelf Getz Ice Shelf
This massive ice cliff was photographed at
Ice shelves surround about 44 percent of the continent the seaward edge of the Riiser-Larsen Ice In front of it, emperor penguins line of Antarctica and cover an area of some 600,000 square Shelf. up to enter the water at Atka Bay, miles (1.5 million square km). The largest is the Ross on the Weddell Sea. Ice Shelf, also called the Great Ice Barrier, discovered by Sir James Clark Ross (see panel, above). It is as large as mainland France, with an area of about 190,000 square miles (500,000 square km) and is fed by seven different ice streams. The second largest, the Ronne–Filchner Ice Shelf, covers about 160,000 square miles (430,000 square km). About 15 or so other ice shelves are dotted around the edge of the continent. Since 1995, a few of the smaller ice shelves around the Antarctic Peninsula, including parts of the Larsen Ice Shelf, have disintegrated, most probably as a result of ocean warming (see p.487).
Ross Sea ICE-SHELF LOCATIONS
Voyeykov Ice Shelf
Sulzberger Ice Shelf
The two largest ice shelves—the Ross and Ronne–Filchner ice shelves—sit on either side of west Antarctica.
Cook Ice Shelf
OCEAN ENVIRONMENTS
Structure and Behavior Every ice shelf is anchored to the sea floor (ending at a point called the grounding line) and has a front part that floats. The front part is usually 330–3,300 ft (100–1,000 m) thick, though only about one-ninth protrudes above water. The back of an ice shelf is fixed while the front part moves up and down with the tides, creating stresses that can lead to the formation of cracks. Overall, there is a gradual movement of ice from the rear to the front of an ice shelf, from where large tabular icebergs occasionally calve. There is sometimes also a slow upward migration of ice, due to seawater freezing to the bottom of a shelf and the ice on the CALVING SHELF The front part of an ice shelf will upper surface melting and evaporating in sometimes break up and the pieces summer. Even deposits from the sea floor drift off as tabular icebergs. Each under an ice shelf are sometimes brought piece visible here has a surface area of several square miles. to the surface by this mechanism.
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GAINS AND LOSSES
An ice shelf gains ice from glaciers flowing into its landward end, from new snowfall, and from seawater freezing to its undersurface. It loses ice by iceberg calving, by some summer melting of its upper surface and through evaporation, and by melting of part of on its undersurface.
In fl o wf rom
Sea Level 1000m (3280ft)
2000m (6560ft)
Gains in ice Losses of ice
Summer evaporation From ponds on surface
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Iceberg calving
Tide Cracks
Icebergs c ic
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Floating ice shelf
Sea level rises and falls with tide Grounded ice
Freezing of seawater onto underside of ice shelf Melting of ice at depth Grounding line
ICE SHELVES
Surface and Interior
Beneath the Ice Shelves
The upper surfaces of Antarctic ice shelves are inhospitable places. For most of the year, cold air streams called katabatic winds blow down from the Antarctic Ice Sheet and over the ice shelves. The surface of the ice is not flat, but is shaped by the winds into a series of ridges and troughs, called sastrugi. These are typically covered in a snow blanket. In some areas, the surface is littered with rocks from the input glacier or glaciers, or even with material that has been carried upward from the sea floor by vertical movement. In summer, small ponds form on some ice shelves and provide a home for various types of microscopic organisms. Internally, an ice shelf usually contains some tide-induced cracks and crevasses.
Underneath the Antarctic ice shelves are extensive bodies of water that are some of the least explored regions on Earth. Seawater is thought to circulate constantly here, caused partly by new ice formation underneath and around the ice shelves. As new ice forms, it “rejects” salt, making the surrounding seawater denser. This causes the seawater to sink, and helps drive the circulation. Recent attempts have been made to explore these areas, using robotic submarines to take measurements. Little is known about the organisms that LIFE UNDER THE ICE live here, although in 2005 a community Organisms such as starfish and of clams and bacterial mats was found on worms live in shallow water the sea floor under the Larsen B Ice Shelf around the edge of Antarctica, and possibly under the ice shelves. after it broke up (see p.487).
CAVE INSIDE AN ICE SHELF
In summer, the internal cracks and crevasses in an ice shelf may enlarge to form caves as some of the ice melts.
new ice ice platelets rise as density decreases
low-salinity water
annually reforming fast ice
marine ice is found beneath sea level
ice shelf
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melting zone
SEAWATER CIRCULATION
A continuous circulation of seawater is thought to occur under large ice shelves, driven by sea-ice formation on its undersurface and partial melting at depth.
high-salinity water grounding line ice pump driven by salt rejection
OCEAN ENVIRONMENTS
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POLAR OCEANS
Icebergs
ICEBERGS ARE HUGE, FLOATING
OCEAN ENVIRONMENTS
chunks of ice that have broken off, or been calved, from the edges of large glaciers and ice shelves. These chunks range from car-sized objects to vast slabs of ice that are bigger than some countries. It is estimated that each year 40,000 to 50,000 substantial icebergs are calved from the glaciers of Greenland. A smaller number of gigantic icebergs break off the ice shelves around Antarctica. Surface currents carry icebergs away from their points of origin into the open ocean, where they drift and slowly melt. They can last for years and are a considerable danger to shipping.
Sizes and Colors Icebergs include pieces of ice that are hundreds of square miles in area, down to ones the size of houses (bergy bits) or cars (growlers). Tabular icebergs may rise to a height of up to 200 ft (60 m) above the sea surface and extend underwater to a depth of up to 1,000 ft (300 m). Most icebergs appear white because of the light-reflecting properties of air bubbles trapped in the ice. Those made of dense, bubble-free ice absorb all but the shortest (blue) light wavelengths and so have a vivid blue tint. Occasionally, icebergs roll over and expose a previously submerged section to view, which appears aqua green because of algae growing in the ice.
Iceberg Properties
ICEBERG PROPORTIONS
Because pure ice is 90 percent as dense as seawater, an iceberg made entirely of ice will have only 10 percent of its mass visible above water.
Icebergs consist principally of frozen fresh water, with no salt content. This is because they originate not from seawater but from glaciers or ice shelves (floating glaciers), and glaciers themselves come from compacted snow. Typically, an iceberg has a temperature of about -4 to 5˚F (-15 to -20˚C) at its core and 32˚F (0˚C) at its surface. In addition to ice, some icebergs contain rock debris. This is material that has fallen onto the parent glacier from surrounding mountains, or frozen to the glacier’s edges, and eventually becomes incorporated into the ice. An iceberg’s rock load affects its buoyancy. An iceberg with a high rock content may float up to 93 percent submerged.
RANGE OF SHAPES
Icebergs come in a range of shapes including tabular (flat-topped), domed, pinnacled or pyramidal, wedge-shaped, and various irregular shapes, as shown here. TABULAR
PINNACLED
IRREGULAR
DOMED
ICEBERGS
North Atlantic Icebergs
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HUMAN IMPACT
ICEBERG DETECTION
Most icebergs seen in the north Atlantic begin as snow falling on Greenland. This snow eventually becomes ice, which over thousands of years is transported from the Greenland ice sheet down to the sea as glaciers. Icebergs calved from ARCTIC OCEAN the glaciers on the west coast of Greenland (and many from Ellesmere Greenland Island G the east coast) move into Baffin Sea R E Bay. The Labrador Current E N carries these icebergs southeast, ICELAND Humboldt past Newfoundland, into the Hayes north Atlantic. There, most of the icebergs rapidly melt, Baffin Jakobshavn but a few reach as far south as Bay 40˚N—around the same latitude as New York and Lisbon. Arctic circle
Because of their threat to shipping, north Atlantic icebergs are monitored by the US Coast Guard. Information on iceberg sightings, obtained by aircraft and ships, is fed into a computer along with ocean-current and wind data. The future movements of the icebergs are then predicted so that ships can be warned. The southernmost iceberg ever spotted in the Atlantic was only 155 miles (250 km) from Bermuda at 32˚N.
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These massive icebergs were calved from Breidamerkurjökull, a glacier in Iceland, and form a surreal tourist attraction, drifting in the glacial lagoon.
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Ice Rafting
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All Southern Ocean icebergs have ANTARCTICA broken off one of the ice shelves that Minimum extent of surround Antarctica (see p.192). Most sea ice start off as extremely large, tabular e eb icebergs—satellite monitoring of their of Ic Maximum Limit extent of drift tracks has provided useful information C L E A sea ice RA about Southern Ocean currents. After N ST U A calving, these icebergs drift westward around Antarctica in a coastal current (the East Wind Drift). A few are carried in an eastward direction by DISTRIBUTION the Antarctic Circumpolar Current. In extreme cases, The approximate limit of they drift further, reaching as far north as 42˚S in the iceberg drift from Antarctica is shown by the red dotted Atlantic Ocean. The largest Southern Ocean iceberg line. Most Southern Ocean ever recorded measured 183 miles (295 km) long icebergs remain close to the and 23 miles (37 km) wide. Antarctic Circle at 67˚S.
Icebergs that contain rock debris gradually release this material as they melt, and the debris sinks to the sea floor. Thus rock fragments DIRTY ICEBERG can be transported from The fact that this iceberg contains considerable Greenland, for example, amounts of rock and dust is plain from its to the bottom of the “dirty” appearance. This rock will end up north Atlantic. The on the sea floor as ice-rafted material. process is called ice rafting. By examining sediment samples taken from the ocean floor, scientists can often identify rock fragments that have been transported in this way. Such studies can provide clues about past patterns of iceberg calving and iceberg distribution. For example, they have shown that there were short cold periods during the last ice age, called Heinrich Events, when vast armadas of icebergs were calved and crossed the Atlantic eastward from the coast of Labrador.
OCEAN ENVIRONMENTS
NO DIA IN
Southern Ocean Icebergs
ICELANDIC ICEBERGS
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Most north Atlantic icebergs are calved by glaciers in west Greenland, such as the Jakobshavn and Hayes glaciers.
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ORIGINS AND DISTRIBUTION
NORTH ATLANTIC OCEAN
WRECK OF THE TITANIC
The Titanic’s bow section, of which the upper deck and railings are seen here, is mostly intact, although deeply embedded in the seafloor.
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The Titanic Disaster THE HISTORY OF THE TITANIC
SETTING SAIL
LEAVING SOUTHAMPTON At the time of its launch, the Titanic was the world’s largest passenger liner and also the most opulent. When the ship left Southampton docks on April 10, 1912, it was carrying about 900 crew and 1,300 passengers, including some of the world’s richest and most prominent people.
MEDIA SENSATION The sinking of the Titanic caused shock around the world. This Chicago newspaper dates from April 16, 1912.
DISASTER
The sinking of the ocean liner Titanic on April 15, 1912, in the north Atlantic ranks as one of the worst peacetime maritime disasters in history. It is also arguably the most famous sinking of all time, partly because the ship had been considered unsinkable. A total of more than 1,500 people died in the disaster, while just over 700 survived. The exact sequence of blunders by which the Titanic came to collide with an iceberg has never been fully explained. It is known that during the 12 hours preceding the disaster, messages were sent from other ships that large icebergs lay in the Titanic’s path. However, these messages may not have reached the ship’s bridge. When the collision occurred, the iceberg did not hit the Titanic head-on, but brushed the starboard side. However, this was enough to buckle the hull and dislodge rivets below the waterline, creating leaks into five of the ship’s hull compartments. Although lifeboats were deployed, there were not enough to hold everyone. Furthermore, some were launched before they were full. As a result, about 1,500 people were still on the ship when it sank. Most are thought to have died of hypothermia in the ice-cold waters. In 1985, the wreck of the Titanic was located by an American–French team, by means of an underwater vehicle with a video camera and lights attached. A notable discovery was that the ship had split in two before sinking—the bow and stern were found lying 2,000 ft (600 m) apart, facing in opposite directions.
THE UNSINKABLE SINKS In the early morning of April 15, about 2.5 hours after colliding with an iceberg, the Titanic’s stern rose out of the water as the ship sank. BOB BALLARD Along with French scientist Jean-Louis Michel, American oceanographer Bob Ballard led the team that discovered the wreck of the Titanic on September 1, 1985, at a depth of 12,500 ft (3,800 m).
The Titanic left England on April 10, 1912, bound for New York. After crossing the English Channel, the ship took on additional passengers in France, and also stopped in Ireland the next day, before continuing on its journey. Three days later, on April 14, the ship’s captain altered course slightly to the south, possibly in response to iceberg warnings received over the radio. However, at 11:40 pm, lookouts spotted a large iceberg directly in front of the ship. Despite a frantic avoiding maneuver, the Titanic hit the iceberg, and by 2:20 am, the ship had sunk.
LIFEBOAT WINDLASS This piece of deck machinery was barely recognizable under a covering of rusticles (nodules containing a mixture of iron compounds and microbes that feed on wrought iron). BANKNOTES Banknotes in surprisingly good condition have been retrieved, including this $5 bill found in the purser’s bag.
departs Southampton 10th April
CANADA
Queenstown
sinks 15th April
New York ATLANTIC OCEAN
ARTIFACTS
Cherbourg
CHINA DISHES Rows of dishes were found lying on the seafloor. Such diverse items as books, watches, and wireless messages have also been retrieved, along with a bronze cherub and hundreds of other objects.
OCEAN ENVIRONMENTS
WAS THIS THE ICEBERG? This photograph, taken six days later in the vicinity of the disaster, shows an iceberg that closely accorded with descriptions provided by survivors.
DISCOVERY OF THE WRECK
First and Last Voyage
198
POLAR OCEANS
Sea Ice
TESTING THE ICE
SEA ICE IS SEAWATER THAT HAS FROZEN
at the ocean surface and floats on the liquid seawater underneath. It includes pack ice—ice that is not attached to the shoreline and drifts with wind and currents—and fast ice, which is frozen to a coast. Sea-ice formation and melting influences the large-scale circulation of water in the oceans. It has important stabilizing effects on the world’s climate, since it helps control the movement of heat energy between the polar oceans and atmosphere. Sea ice strongly reflects solar radiation, so in summer it reduces heating of the polar oceans. In winter, it acts as an insulator, reducing heat loss. Today, scientists are concerned about shrinking sea ice in the Arctic because of its possible effects on climate and wildlife.
Pancake ice, consisting of ice platelets, can be up to 4 in (10 cm) thick. Waves and wind have caused these platelets to collide, hence their curled-up edges.
Formation Seawater starts to freeze when it reaches a temperature of 28.8˚F (-1.8˚C), slightly cooler than the freezing point of fresh water. Sea-ice formation starts with the appearance of tiny needlelike ice crystals (frazil ice) in the water. Salt in seawater cannot be incorporated into ice, and the crystals expel salt. The developing sea ice gradually turns into a thick slush and then, under typical wave conditions, into a mosaic of ice platelets called pancake ice. Subsequently, it consolidates into a thick, solid sheet, through processes such as “rafting” (in which the ice fractures and one piece overrides another) and “ridging” (where lines of broken ice are forced up by pressure). Where ridging occurs, each ridge has a corresponding structure, a keel, that forms on the underside of the ice. Newly formed, compacted sheet ice is called first-year ice and may be up to 12 in (30 cm) thick. It continues to thicken through the winter. Any ice that remains through to the next winter is called multi-year ice.
OCEAN ENVIRONMENTS
HOW ICE FORMS
The stages of sea ice formation vary according to whether the sea surface is calm or affected by waves. A typical sequence in an area of moderate wave action is shown below.
GREASE ICE
PANCAKE ICE
FIRST-YEAR ICE
MULTI-YEAR ICE
Fine ice spicules, called frazils, appear in the water. These coagulate into a viscous soup of ice crystals, called grease ice.
Wave action causes the grease ice to break into slushy balls of ice, called shuga. These clump into platter shapes called pancakes.
The ice pancakes congeal, consolidate, and thicken through processes such as rafting and ridging to form a continuous sheet of ice.
Further thickening, for a year or more, produces multi-year ice. This has a rough surface and may be several yards thick.
SEA ICE
Extent and Thickness
199
DISCOVERY
c le
The extent of sea ice in the polar oceans varies over an annual cycle. About 85 percent of the winter ice that forms in the Southern Ocean melts in summer, and on average this ice only reaches a thickness of a yard or two. In the Arctic, some of the ice lasts for several seasons, and this multi-year ice attains a greater thickness—on average 7–10 ft (2–3 m). In winter, pack ice covers most of the Arctic Ocean. In summer, it shrinks in area by more than two-thirds. In recent years, the summer retreat has been more pronounced, raising fears that summer ice coverage Arct may disappear altogether ic C ir by 2050 or earlier.
USS NAUTILUS In 1958, a US submarine, the USS Nautilus, crossed the Arctic Ocean underneath its cover of sea ice, passing the North Pole on August 3. The crossing proved that there is no sizable land mass in the middle of the Arctic Ocean. The submarine traversed the Arctic from the Beaufort Sea to the Greenland Sea in four days at a depth of about 500 ft (150 m).
ARCTIC SEA ICE COVERAGE
ARCTIC OCEAN
Coverage varies from a winter high of 6 million square miles (15 million square km) to a summer low of less than 1.75 million square miles (4.5 million square km). year-round ice winter sea ice
Gaps in the Ice
ICE LEAD
An ice lead forms when an area of sea ice shears. Stresses from winds and water currents are thought to be the cause. Here, a group of beluga whales swims along a lead.
Even in parts of the polar oceans that are more or less permanently ice-covered, gaps and breaks sometimes appear or persist in the ice. These openings vary greatly in size and extent and have different names. Fractures are extremely narrow ruptures that are usually not navigable by boats of any size. An ice lead is a long, straight, narrow passageway that opens up spontaneously in sea ice, making it navigable by surface vessels and some marine mammals. Polynyas are persistent regions of open water, up to a few hundred square miles in area and often roughly circular in shape. They sometimes develop where there is upwelling of warmer water in a localized area, or near coasts where the wind blows new sea ice away from the shore as it forms. ANTARCTIC KRILL
Life Around the Ice
ICEBREAKERS Icebreakers are ships designed for moving through ice-covered environments. An icebreaker has a reinforced hull and a bow shape that causes the ship to ride over sea ice and crush it as it moves forward. The shape of the vessel clears ice debris to the sides and under the hull, allowing steady progress. The most powerful modern icebreaker can advance through sea-ice up to 9 ft (2.8 m) thick.
WEDDELL SEAL
The Weddell seal, found only in the Antarctic, is one of nine seal species that inhabit polar oceans. Weddell seals never stray far from sea ice.
OCEAN ENVIRONMENTS
HUMAN IMPACT
These crustaceans form an important
part of the food chain in the Southern Life thrives around sea ice. One reason for this is that as ice forms, Ocean, where they congregate in salt is expelled into the seawater, causing it to become denser and dense masses. sink. This forces nutrient-laden water to the surface. In summer, the combination of nutrients and sunlight encourages the growth of phytoplankton, which provide a rich food source. These organisms form the base of a food chain for fish, mammals, and birds. In the Arctic, sea ice provides a resting and birthing place for seals and walruses and a hunting and breeding ground for polar bears and Arctic foxes. In the Antarctic, it supports seals and penguins. Breaks in the ice are vital to this wildlife. Seals, penguins, and whales rely on them for access to the air, while polar bears hunt near them. Decreases in Arctic sea ice would drastically shrink some habitats, pushing them toward extinction.
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POLAR OCEANS
Polar Ocean Circulation THE ARCTIC AND SOUTHERN OCEANS
each have their own unique patterns of water flow, which link in with the rest of the global ocean circulation. These flows are driven partly by wind and partly by various factors that influence the temperature and salinity of the surface waters in these oceans—including seasonal variations in air temperature and sea ice coverage, and large inflows of fresh water from rivers. Although driven by similar influences, the significantly different water-flow patterns of these two oceans are largely due to the fact that the Arctic Ocean is encircled by land, whereas the Southern Ocean surrounds a frozen continent.
Arctic Surface Circulation The upper 170 ft (50 m) of the Arctic Ocean is affected by currents that keep it in constant motion. There are two main components to this circulation (see p.424–25). In a large area north of Alaska, there is a slow, circular motion of water called the Beaufort Gyre. This clockwise movement is wind-generated and completes one rotation every four years. The second component, the Transpolar Current, is driven by a vast quantity of water discharged into the Arctic Ocean from Siberian rivers.
MOUTH OF THE LENA RIVER
OCEAN ENVIRONMENTS
The Lena flows across Siberia and discharges 100 cubic miles (420 cubic km) of water into the Arctic Ocean every year.
CIRCULATION AND FEEDING
The Southern Ocean meets warmer water at the Antarctic Convergence, creating a biologically rich feeding area for whales, including these humpbacks.
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Arctic Deep-water Circulation
MISTY SEAS
In the deeper waters of the Arctic, there is a slow circulation of cold, dense water. This circulation is restricted by the structure of the Arctic Ocean, which consists of a central deep basin (the Arctic Basin), bisected by several underwater ridges and surrounded on most sides by shallow continental shelf. Only on the Atlantic side is there a connection between the deep waters of the Arctic and deep ocean waters to the south. On the opposite side, the connection with the Pacific is via the shallow and narrow Bering Strait. What little circulation of water occurs in the Arctic Basin mostly involves influxes of Atlantic water at various depths to the north of Russia, and outflows around Greenland.
PA
C
ARCTIC BASIN CIRCULATION
IF
IC
OC
E
AN
tic Circle Bering Arc Strait ARCTIC OCEAN
Atlantic water enters to the north of Russia and Pacific water via the Bering Strait. As it cools, some of the Atlantic water becomes denser and dips far below the sea ice cover, where it flows around slowly. The main outflow is to the east of Greenland.
The seas around the Antarctic Convergence are prone to mists. Here, a cruise liner approaches a channel just south of the Convergence.
The Antarctic Convergence The Antarctic Convergence is a region of the Southern Ocean encircling Antarctica, located roughly at latitude 55˚S (but deviating from this in places), where cold, northward-flowing waters from Antarctica sink beneath the relatively warmer waters to the north. At the Convergence, there is a sudden change in surface ocean temperature of 5–9˚F (3–5˚C) as well as alterations in the chemical composition of seawater. As a result, the Convergence forms a barrier to the movement of animal species, and the groups of marine animals found on either side of it are quite different. This is a turbulent area. The meeting of different water masses brings dissolved nutrients from the sea bed to the ocean surface. This acts as a fertilizer, encouraging the growth of plankton during the Southern Hemisphere summer. PHYTOPLANKTON
atlantic water pacific water
In summer, massive blooms of phytoplankton occur around the Antarctic Convergence, forming the base of a productive food chain.
Southern Ocean Circulation
AT L A N T I C OCEAN
ALBATROSSES
These black-browed and gray-headed albatrosses inhabit the biologically productive Southern Ocean.
In the Southern Ocean, surface waters move under the influence of two wind-driven currents. Off the coast of Antarctica, the Antarctic Coastal Current carries water from east to west around Antarctica. Several hundred miles north, the Antarctic Circumpolar Current (ACC) moves water in the opposite direction, from west to east, and pushes the Antarctic waters northward. The ACC is a major ocean current that connects the Pacific, Atlantic, and Indian oceans and isolates Antarctica from the warmer ocean currents to the north. In the Southern Ocean, an important movement of water also occurs deep down. In an area near Antarctica, masses of dense, salty water form as salt is rejected from seawater as it freezes. This cold water sinks and moves north into the southern Atlantic.
FRIDJTOF NANSEN The Norwegian explorer and scientist Fridjtof Nansen (1861–1930) is most famous for his Arctic voyage of 1893–1895 on a specially built wooden ship called the Fram. Nansen deliberately allowed the Fram to drift across the Arctic Ocean locked in ice, and in doing so proved the existence of the surface current now called the Transpolar Current. In 1895, setting off with one companion from the Fram, Nansen walked and skied to within 400 miles (640 km) of the North Pole, closer than anyone else up to that time.
OCEAN ENVIRONMENTS
PEOPLE
OCEAN LIFE
BY FAR THE LARGEST HABITAT on Earth,
the oceans are more accurately seen as a great range of environments as disparate as mangrove swamps and deep-sea vents. Living organisms have found a place to take hold in every ocean environment, even the deepest trenches, more than 6 miles (10 km) beneath the surface. Ocean life teems with greatest abundance and variety in the sunlit surface waters. Here, microscopic plants and plantlike organisms, the phytoplankton, fuel productive communities of organisms right up to top predators, such as killer whales. Life began in the oceans, and they were the site of many groundbreaking steps in evolution. Tracing the history of this evolution puts into context the astonishing variety of today’s marine life.
IN TR OD U C TI ON TO O C E A N L IF E KELP FOREST COMMUNITY
Ocean life develops into one of a number of characteristic communities, according to physical conditions. Here in cool, shallow water, a canopy of kelp towers above an undergrowth of encrusting red seaweed, while an eagle ray and smaller fish find shelter among the kelp fronds.
206
INTRODUCTION TO OCEAN LIFE
Classification BY CLASSIFYING ORGANISMS AND FITTING
them into a universally accepted framework, scientists have created a massive reference system that accommodates all forms of life. Over 2 million organisms have been described, of which about 16 percent live in the oceans. The marine proportion is likely to increase because many new species continue to be discovered annually, particularly in the deep ocean.
WHAT IS A SPECIES? A species is the basic unit of classification. One commonly accepted definition of a species is a group of living organisms that have so many features in common that they can interbreed and exchange genes in natural conditions. This definition cannot be applied to fossil species. It also does not work for Bacteria and Archaea and there can be no one universal definition. Other factors such as geographical isolation and DNA (see below) are also important.
LINNAEAN HIERARCHY
Principles of Classification
Linnaeus used a hierarchy of ranked categories of increasing exclusiveness. Today’s expanded system includes many ranks, from domain down to species. Below is an example of a series of ranked categories, illustrating those that classify the common dolphin.
Classification helps us make sense of the natural world by grouping organisms on the basis of features that they share. It gives scientists a clear and accurate understanding of the diversity of life, and because everyone uses the same system, the knowledge is accessible on a worldwide basis. The hierarchical system devised by the Swedish scientist Carolus Linnaeus (see panel, left) in the 18th century still forms the basis of today’s classification. Each species is identified with a unique two-part scientific name (made up of the genus and species names), then categorized in a series of ever-larger groupings. However, as our knowledge increases, it is often necessary to revise the groups. Sometimes, this leads to subdivision of categories, for example phylum Arthropoda has been split into four subphyla. Many new species are discovered each year but describing and publishing them is laborious and time-consuming.
DOMAIN Eucarya Includes all Eukaryotes—organisms that have complex cells with distinct nuclei. Only bacteria and archaea fall outside this domain. KINGDOM Animalia Includes all animals—multicellular eukaryotes that need to eat food for energy. All animals are mobile for at least part of their lives. PHYLUM Chordata Includes all chordates—animals possessing a notochord. In most cases, the notochord is replaced before birth by the backbone. CLASS Mammalia Includes all mammals—air-breathing chordates that feed their young on milk. The jaw is made up of a single bone. ORDER Cetartiodactyla Includes all cetaceans (whales and dolphins)— marine mammals that have a tail with boneless, horizontal flukes for propulsion. FAMILY Delphinidae Includes all dolphins (a subgroup of toothed cetaceans) with beaks and 50–100 vertebrae. The skull lacks a crest. GENUS Delphinus Includes a few colorful, oceanic dolphins with 40–50 teeth on each side of the jaw. These dolphins form large social groups. SPECIES Delphinus delphis Specifies a single type of dolphin with a V-shaped black cape under the dorsal fin and criss-cross hour-glass patterning on its sides.
The Evidence In the past, scientists could identify and classify organisms only by studying anatomy, by looking at form, function, and embryological development (animals only), and by examining the fossil record. Recently, scientists have also been able to investigate organisms by looking at their proteins and their DNA. DNA is a DETAILED ANATOMY complex molecule whose sequential By making a detailed structure is unique to each organism. anatomical examination of material in museum The relatedness of organisms can collections, scientists be determined by comparing these can distinguish between DNA molecules for shared features. similar organisms and This molecular evidence has led to classify them according to shared characters. many revisions of classification. ANIMALS WITH A SKULL A skull is a derived character that unites all the organisms below. The skull is said to have evolved in their common ancestor.
OCEAN LIFE
Cladistics JAWED VERTEBRATES By the 1950s, although most people used the same system of Animals beyond this point form a clade of classification, the criteria they used for placing organisms in organisms with jaws, again assumed to have categories were often neither measurable nor repeatable. been inherited from a common ancestor. The idea emerged to analyze many characters using an automatic, computer-like process, not only to classify BONY VERTEBRATES organisms, but also to trace their evolution. All animals beyond this point form a clade This process became known as cladistics, possessing an inherited bony skeleton, not shared by sharks, lampreys, or hagfish. and it is a widely used technique today. LAMPREY A cladistic analysis examines a wide selection HAGFISH FISH CLADOGRAM of characters shared by a study group of RAY-FINNED FISH This simplified Below is a clade of fish with fins organisms. It finds the most likely pattern of cladogram indicates made up of radiating bones only, evolutionary changes that link the organisms, just three of the steps without the limblike lobes of used to classify fish. involving the least number of steps (evolutionary lobe-fins, or limbs of tetrapods. CARTILAGINOUS FISH Clades include all the branching points). It then arranges the organisms descendants of a common in a tree diagram (cladogram) that reflects their ancestor, so some new groups, such relationships. A cladogram is made up of nested as “lobe-finned fish and tetrapods” groups called clades. A clade encompasses all result, since all tetrapods (land LOBE-FINNED FISH RAY-FINNED FISH the descendants of the group’s common ancestor. vertebrates) descend from lobe-fins. AND TETRAPODS
CLASSIFICATION although the numbers of classes and species cited include all organisms within the group whether they are marine or not. Some groupings, such as fish, are shown in dotted lines because although they are useful categories, they are not true taxonomic groups. Others, such as bottomliving phyla and planktonic phyla, are ecological groupings and do not reflect taxonomy or evolutionary history.
Marine Life THE CLASSIFICATION FRAMEWORK USED in this book is shown on the following three pages. In this framework, all living things are divided into three domains. Within domains, only the marine groups are shown,
BACTERIA DOMAIN
Bacteria
PHYLA
ARCHAEA About 80
SPECIES
Many millions
DOMAIN
Archaea
PHYLA
EUKARYOTES THIS DOMAIN INCLUDES ALL ORGANISMS
that have cells with a nucleus and other complex structures not seen in prokaryotes (bacteria, archaea). The eukaryotes comprise protists, chromists, plants, fungi, and animals.
EUKARYOTES 2
SPECIES
Probably millions
DOMAIN
Eucarya
GREEN SEAWEEDS CLASS
Ulvophyceae
ORDERS
Dinoflagellates Myzozoa
INFRAPHYLUM
CLASSES
4
SPECIES
2,436
About 10
SPECIES
8,699
CLASSES
3
SPECIES
4,000
Foraminiferans CLASSES
3–5
SPECIES
6,616
CLASSES
2
SPECIES
CLASSES
8
SPECIES
260,684
SUPERCLASS
Angiospermae
ORDERS
30
SPECIES
260,000
Fungi Fungi
PHYLA
5
SPECIES
46,574
KINGDOM
Animalia
PHYLA
About 30
SPECIES
Over 1.5 million
progresses from organisms with simple body plans and systems, such as sponges, to the most complex phylum, chordates (pp.208–209), which contains humans. Each phylum represents a distinct body plan.
Porifera
CLASSES
4
SPECIES
About 8,700
CLASSES
5
SPECIES
10,886
ORDERS
10
SPECIES
7,095
ORDERS
3
SPECIES
186
ORDERS
2
SPECIES
41
ORDERS
7
SPECIES
3,516
SPECIES
48
258
CNIDARIANS
Ochrophyta
PHYLUM
Ochrophyta
CLASSES
20
ORDERS
12
SPECIES
100,000
ORDERS
4
SPECIES
490
ORDERS
23
SPECIES
2,053
DIATOMS
SPECIES
5,006
Phaeophyceae
Cubozoa
HYDROIDS CLASS
+ SEVERAL MORE PHYLA
Scyphozoa
BOX JELLYFISH CLASS
BROWN SEAWEEDS
Anthozoa
JELLYFISH CLASS
Chrysophyceae
Cnidaria
CORALS AND ANEMONES CLASS
Bacillariophyceae
GOLDEN YELLOW ALGAE
CLASS
13,365
FLOWERING PLANTS
PHYLUM
Haptophyta
CLASS
SPECIES
SPONGES
Coccolithophorids
CLASS
3
THE FOLLOWING LIST OF ANIMAL PHYLA
Foraminifera
PHYLUM
1,500
Animals
Radiozoa
PHYLUM
Trachaeophyta
DIVISION
KINGDOM CLASSES
Radiolarians
PHYLUM
SPECIES
2 million
+ THREE NON-MARINE DIVISIONS
Dinoflagellata
Ciliophora
PHYLUM
SPECIES
+ SEVEN NON-MARINE CLASSES
Ciliates PHYLUM
8–9
At least 8
CLASSES
VASCULAR PLANTS
is complex, difficult, and constantly in flux. Many important marine plankton groups are collectively referred to as chromists. Others are instead plants or protozoans (kingdom Protozoa).
PHYLUM
Bryophyta
DIVISION
THE CLASSIFICATION OF SINGLE-CELLED ORGANISMS
KINGDOMS
+ SIX MORE CLASSES OF MAINLY MICROSCOPIC GREEN ALGAE
MOSSES
Chromists
207
Hydrozoa
STALKED JELLYFISH CLASS
Staurozoa
ORDERS 1
Plants KINGDOM
Plantae
DIVISIONS
8
SPECIES
315,000
PLANTS COMPRISE EIGHT DIVISIONS, only three of which have truly marine species and are included in this book. Mosses (Bryophyta) are additionally included because a few of them live in the intertidal zone.
DIVISION
Rhodophyta
CLASSES
2 or more
SPECIES
6,394
CLASSES
About 8
SPECIES
5,426
Chlorophyta
SPECIES
200
Prasinophyceae
COMB JELLIES PHYLUM
Ctenophora
CLASSES
2
SPECIES
187
PHYLUM
Chaetognatha
CLASSES
1
SPECIES
131
CLASSES
2
SPECIES
2,014
ROTIFERANS
GREEN ALGAE (MICROSCOPIC) CLASS
with the ocean currents in the plankton and are grouped here on this basis. The Ctenophora and Chaetognatha contain so few species that they are known as minor phyla.
ARROW WORMS
GREEN SEAWEEDS AND ALGAE DIVISION
THE FOLLOWING THREE PHYLA FLOAT
ORDERS
3
PHYLUM
Rotifera
OCEAN LIFE
RED SEAWEEDS
PLANKTONIC PHYLA
208
INTRODUCTION TO OCEAN LIFE
FLATWORMS
CHITONS
Polyplacophora
CLASS
PLATYHELMINTHES PHYLUM
Platyhelminthes
CLASSES
6
SPECIES
20,000
CLASSES
2
SPECIES
430
XENACOELMORPHA PHYLUM
Xenacoelomorpha
Arthropoda
PHYLUM
Nemertea
CLASSES
2
SPECIES
1,358
CLASSES
2
SPECIES
15,000
Annelida
4
CRUSTACEANS
Crustacea
CLASS
Branchiopoda
CLASS
Maxillopoda
CLASS
MEMBERS OF THE FOLLOWING PHYLA all live in or on the ocean floor. The list is not comprehensive—the following phyla are among those not included: Entoprocta, Acanthocephala, Placozoa, and Cephalorhyncha.
SPOON WORMS PHYLUM
Echiura
CLASSES
LAMP SHELLS PHYLUM
Brachiopoda
CLASSES
HORSESHOE WORMS PHYLUM
Phoronida
CLASSES
PEANUT WORMS PHYLUM
Sipuncula
CLASSES
3
SPECIES
900
ORDERS
27
SPECIES
16,589
ORDERS
5
SPECIES
7,462
36,759
ORDERS
Cycliophora
CLASSES
Gastrotricha
CLASSES
Nematoda
2
SPECIES
SPECIES
1
SPECIES
1
SPECIES
CLASSES
2
SPECIES
PRIAPULA WORMS AND MUD DRAGONS PHYLUM
Cephalorhyncha Tardigrada Hemichordata
16
15
SPECIES
FAMILIES
17
SPECIES
480
ISOPODS ORDER Isopoda
FAMILIES
94
SPECIES
11,515
AMPHIPODS ORDER Amphipoda
FAMILI ES
119
SPECIES
10,158
KRILL ORDER Euphausiacea
FAMILIES
2
SPECIES
86
SPECIES
15,000
LOBSTERS, CRABS, AND SHRIMPS ORDER Decapoda FAMILIES 105
147 2 847 20,000
SUBPHYLUM
CLASS
Chelicerata
CLASSES
SPECIES
236
CLASSES
3
SPECIES
1,000
SPECIES
130
3
CLASSES
CAUDOFOVEATES CLASS
Caudofoveata
Solonogasters
Monoplacophora Scaphopoda
73,682
SPECIES
131
ORDERS
4
SPECIES
273
ORDERS
1
SPECIES
30
Merostomata Pycnogonida
SUBPHYLUM
70,000
ORDERS
1
SPECIES
4
ORDERS
1
SPECIES
1,342
Hexapoda
CLASSES
4
SPECIES
About 1.11 million
ORDERS
29
SPECIES
1.1 million
BRYOZOANS PHYLUM
1
SPECIES
571
OCEAN LIFE
Bivalvia
17
SPECIES
ORDERS
16
SPECIES
61,682
ORDERS
9
SPECIES
816
ORDERS
GASTROPODS CLASS
Gastropoda
CEPHALOPODS CLASS
Cephalopoda
9,209
CLASSES
3
Echinodermata
CLASSES
5
SEA LILIES AND FEATHER STARS Crinoidea
SPECIES
6,085
Asteroidea Ophiuroidea
SPECIES
638
ORDERS
8
SPECIES
1,851
ORDERS
2
SPECIES
2,074
ORDERS
16
SPECIES
999
ORDERS
6
SPECIES
1,716
SEA URCHINS CLASS
Echinoidea
SEA CUCUMBERS KINGDOM
Holothuroidea
7,278
4
BRITTLESTARS CLASS
SPECIES
ORDERS
STARFISH CLASS
BIVALVES CLASS
Bryozoa
ECHINODERMS
CLASS
ORDERS
Insecta
+ 1 OTHER NON-MARINE SUBPHYLUM, MILLIPEDES AND CENTIPEDES (MYRIAPODA)
PHYLUM
TUSK SHELLS CLASS
SPECIES
1
MONOPLACOPHORANS CLASS
8
ORDERS
SOLENOGASTRES CLASS
71,004
SPECIES
HEXAPODS
CLASS
MOLLUSKS Mollusca
SPECIES
12
INSECTS
PHYLUM
13
ORDERS
SEA SPIDERS CLASS
CLASSES
Arachnida
HORSESHOE CRABS CLASS
4
PTEROBRANCH WORMS AND ACORN WORMS PHYLUM
393
CLASSES
WATER BEARS PHYLUM
ORDERS
SPIDERS, SCORPIONS, TICKS, AND MITES
ROUND WORMS PHYLUM
1
SPECIES
197
6
MANTIS SHRIMPS ORDER Stomatopoda
CLASS
CHELICERATES
GASTROTRICHS PHYLUM
3
SPECIES
MALACOSTRACANS Malacostraca
About 1.25 million
+ 10 MORE MINOR ORDERS
CYCLIOPHORANS PHYLUM
1
Ostracoda
SPECIES
61,710
MUSSEL SHRIMPS
BOTTOM-LIVING PHYLA
970
SPECIES
SPECIES
CLASSES
BARNACLES AND COPEPODS
SEGMENTED WORMS PHYLUM
SUBPHYLA
WATER FLEAS AND RELATIVES
RIBBON WORMS
3
ARTHROPODS
SUBPHYLUM
PHYLUM
ORDERS
CLASSIFICATION
209
CHORDATES PHYLUM
Chordata
3
SUBPHYLA
SPECIES
64,618
THE VERTEBRATES DOMINATE PHYLUM CHORDATA.
The remaining two, much smaller, subphyla are united with vertebrates by the presence of the rodlike notochord, which becomes the backbone before birth in vertebrates. TUNICATES (SEA SQUIRTS AND RELATIVES) SUBPHYLUM
Tunicata
4
CLASSES
LANCELETS SUBPHYLUM
Cephalochordata
3,026
CLASSES
1
SPECIES
30
CLASSES
10
SPECIES
61,562
VERTEBRATES SUBPHYLUM
SPECIES
CLINGFISH ORDER Gobiesociformes
SPECIES
162
PIPEFISH AND SEAHORSES ORDER Syngnathiformes SPECIES 364
NEEDLEFISH ORDER Beloniformes
SPECIES
266
SCORPIONFISH AND FLATHEADS ORDER Scorpaeniformes SPECIES 1,649
SILVERSIDES ORDER Atheriniformes
SPECIES
344
PERCHLIKE FISH ORDER Perciformes
SPECIES
11,061
SQUIRRELFISH AND RELATIVES ORDER Beryciformes SPECIES 161
FLATFISH ORDER Pleuronectiformes
SPECIES
796
DORIES AND RELATIVES ORDER Zeiformes
PUFFERS AND FILEFISH ORDER Tetraodontiformes
SPECIES
437
SPECIES
33
STICKLEBACKS AND SEAMOTHS ORDER Gasterosteiformes SPECIES 29 + 16 MORE ORDERS
Vertebrata
HAGFISH HAVE AT TIMES
been excluded from the vertebrates because they have only vestiges of a vertebral column. However, recent molecular studies confirm they are related to the other jawless fish, the lampreys. Reptiles, birds, and mammals (as well as amphibians, of which there are no marine species) are informally grouped together as tetrapods (Tetrapoda) within the larger group of jawed vertebrates (Gnathostomata), which also include fish.
REPTILES CLASS
Reptilia
ORDERS
4
SPECIES
7,723
FAMILIES
12
SPECIES
300
FAMILIES
44
SPECIES
7,400
FAMILIES
3
SPECIES
23
TURTLES ORDER
Chelonia
SNAKES AND LIZARDS
Squamata
FISH
ORDER
“FISH” IS AN INFORMAL TERM for four classes of animals. Similarly, “jawless fish,” “cartilaginous fish,” and “bony fish” are informal groupings.
CROCODILES
JAWLESS FISH (AGNATHANS)
Crocodilia
BIRDS
HAGFISH CLASS
ORDER
+ 1 NON-MARINE ORDER: THE TUATARAS (SPHENODONTIDA)
Myxini
LAMPREYS CLASS Cephalaspidomorphi
ORDERS
1
SPECIES
79
ORDERS
1
SPECIES
46
CLASS
Aves
ORDERS
29
SPECIES
9,500
IN THIS CLASSIFICATION, the
birds have been divided into 29 orders. Some scientists consider birds to be grouped within the reptiles. WATERFOWL (DUCKS, GEESE, AND SWANS) ORDER
Anseriformes
FAMILIES
2
SPECIES
177
FAMILIES
1
SPECIES
18
FAMILIES
1
SPECIES
5
FAMILIES
4
SPECIES
142
FAMILIES
1
SPECIES
23
FAMILIES
6
SPECIES
65
FAMILIES
6
SPECIES
119
FAMILIES
5
SPECIES
333
FAMILIES
18
SPECIES
385
FAMILIES
9
SPECIES
230
ORDERS
27
SPECIES
PENGUINS
SHARKS, RAYS, AND CHIMAERAS SHARKS, SKATES, AND RAYS CLASS
Elasmobranchii
ORDER
DIVERS AND LOONS
ORDERS
13
SPECIES
FAMILIES
34
SPECIES
1,241
9
523
Gaviiformes
ORDER
Procellariiformes
GREBES
SKATES AND RAYS ORDERS
ORDER
ALBATROSSES AND PETRELS
SHARKS ORDERS
Sphenisciformes
4
FAMILIES
17
SPECIES
718
ORDER
Podicipediformes
PELICANS AND RELATIVES
CHIMAERAS CLASS
ORDER
Holocephali
ORDERS
1
SPECIES
49
HERONS AND RELATIVES ORDER
ORDER
LOBE-FINNED FISH ORDERS
Actinopterygii
Acipenseriformes
ORDERS
SPECIES
Elopiformes
SPECIES
45
SPECIES
31,282
9
LIGHTFISH AND DRAGONFISH ORDER Stomiiformes SPECIES 426
SPECIES
219
Albuliformes
SPECIES
13
Anguilliformes
SPECIES
908
LANTERNFISH AND RELATIVES ORDER Myctophiformes SPECIES 252
SPECIES
263
Charadriiformes
VELIFERS, TUBE-EYES, RIBBONFISH
HERRINGS AND RELATIVES ORDER Clupeiformes SPECIES 399
COD FISH AND RELATIVES ORDER Gadiformes SPECIES 610
MILKFISH ORDER Gonorhynchiformes
SPECIES
37
TOADFISH AND MIDSHIPMEN ORDER Batrachoidiformes SPECIES 83
CATFISH AND KNIFEFISH ORDER Siluriformes
SPECIES
3,604
CUSK EELS ORDER Ophidiiformes
321
ANGLERFISH ORDER Lophiiformes
SPECIES
ORDER
Lampriformes
ORDER
Coraciiformes
+ 18 NON-MARINE ORDERS
MAMMALS CLASS
Mammalia
5,500
SPECIES
SPECIES
SPECIES
25
mammal orders are listed here. The pinnipeds (seals, sea lions, and walruses), until recently classified as order Pinnipeda, do not form a natural group, and have been placed within order Carnivora (cats, dogs, bears, otters, and relatives). The 27 mammal orders includes new orders formerly classified as marsupials. CARNIVORES ORDER
Carnivora
FAMILIES
9
SPECIES
249
FAMILIES
12
SPECIES
85
FAMILIES
2
SPECIES
4
WHALES AND DOLPHINS
Cetacea
531
ORDER
358
SEA COWS ORDER
Sirenia
+ 23 MORE NON-MARINE ORDERS
OCEAN LIFE
SWALLOWERS AND GULPERS ORDER Saccopharyngiformes SPECIES 28
SMELTS AND RELATIVES ORDER Osmeriformes
ORDER
THREE PARTLY OR WHOLLY MARINE
GRINNERS ORDER Aulopiformes
EELS ORDER
8
SALMONS ORDER Salmoniformes
BONEFISH ORDER
SPECIES
28
TARPONS AND TENPOUNDERS ORDER
3
KINGFISHERS AND RELATIVES
STURGEONS AND PADDLEFISHES ORDER
Falconiformes
WADERS, GULLS, AND AUKS
Sarcopterygii
RAY-FINNED FISH SUBCLASS
Ciconiiformes
BIRDS OF PREY
BONY FISH
SUBCLASS
Pelicaniformes
RED SEA REEF
The Red Sea is one of the world’s top 18 coral hotspots. Its colorful reefs are home to an abundance of marine life, including the venomous red lionfish.
211
Biodiversity Hotspots
Tropical coral reefs are popular with divers because they are colorful, shallow and mostly easy to reach. Both with scientists and with many recreational divers involved in collecting data as part of worldwide projects such as Reef Check, we know far more about life on coral reefs than many other ocean habitats. A study in 2002 pinpointed 18 coral reef hotspots (shown in red below). These sites cover 35 percent of the world’s total coral reef area but are home to more than 60 percent of rare and localized reef species, so they are a high conservation priority. An area called the Coral Triangle (outlined in the map below) stretches from the Philippines in the north, to Malaysia, Indonesia, and Timor Leste in the south and west to Papua New Guinea and the Solomon Islands. This giant hot spot supports 600 species of reef-building corals and more than 2000 species of reef fish. ARCTIC OCEAN
ATLANTIC OCEAN
other coral reef areas
Coral Triangle
HIDDEN HOTSPOT BURIED RICHES
abundance of rare and endemic species
SABA BANK A 2006 survey of this atoll in the Caribbean Netherlands found a high species biodiversity. The area was declared a national park in 2012. NEW TO SCIENCE The Saba Bank study discovered a seven-spined goby living on the seabed. It is a new species, and probably a new genus.
GUADALUPE SEAMOUNT Up to a third of species of seaweed, plants, and animals on isolated seamounts may be unique (or endemic) to that seamount, having evolved there over millions of years.
SAMPLING SEDIMENTS Mud cores collected from the seabed can contain thousands of species of microorganisms. Deepsea sediment was once thought to be like a desert but this hidden biodiversity proves otherwise.
OCEAN LIFE
SOUTHERN OCEAN
BENEATH THE SURFACE Underwater, however, Loch Carron is as full of life as any tropical coral reef. Animals include soft corals, dahlia anemones, and brittlestars.
FISH HAVEN Soldierfish and snappers gather at a seamount in the Indian Ocean. The upwelling of nutrient-rich currents around seamounts makes them “oases” of the ocean.
PACIFIC OCEAN
INDIAN OCEAN
LOCH CARRON The northwest Highlands of Scotland may be scenic, but very little biodiversity is found in the harsh, rocky landscape that surrounds Loch Carron.
CARIBBEAN TREASURE CHEST
Coral Reef Hotspots
TYPES OF HOTSPOTS
SEAMOUNT COMMUNITIES
Many people have heard of biodiversity hotspots, particularly in the context of documentaries about ocean life. These sites are very popular with filmmakers for the variety of life they exhibit. However, the term is a slight misnomer. Strictly speaking, such sites are “species diversity hotspots,” places where the largest number of species are concentrated in a small area. Identifying such hotspots helps conservationists to decide where protected areas should be set up. However, places where species diversity is low, such as the ocean trenches, are also important because of the remarkable animals that live there. The problem is that too little is known about the deep ocean for scientists to be sure where the highest species diversity occurs beyond the shallow layer accessible to human divers. However, worldwide assessments have been made of the distribution and richness of coral reefs (see p.152 and below), seagrass beds (see p.146), and mangrove swamps (see p.130). Hot spots where there are many different species of sea turtles, sharks, and open-ocean fish have been found near islands, sea mounts, and shelf breaks. A major tenyear project called the World Ocean Census, completed in 2010, collected a huge amount of new data on where species live in the ocean. Many thousands of new species were documented, and the work of describing these is ongoing. Marine scientists from more than 80 countries took part, and many new hot spots were found, including several important sea mounts.
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INTRODUCTION TO OCEAN LIFE
Cycles of Life and Energy ALL LIFE DEPENDS ON ORGANISMS
that harness energy from either chemicals or the Sun to produce food. These organisms, whether phytoplankton, seaweeds, or bacteria, are called primary producers and form the first link of a food chain. This first link is just one point in a cycle that processes chemical energy and nutrients through the entire community of life in an ecosystem, into the physical environment, and back again.
Energy Flow As each organism in an ecosystem is eaten in turn by the next organism in the food chain, food energy flows from prey to consumer. The primary producers—the organisms such as diatoms and bacteria at the beginning of the food chain—are eaten by organisms called primary consumers, which are eaten by secondary consumers, and so on to top predators—animals not preyed upon by anything else. In land-based ecosystems, the total mass of organisms at each succeeding food-chain level decreases, leaving very few top predators. However, in marine ecosystems with phytoplankton as producers, the mass is greatest at the primary consumer level. This is possible because phytoplankton grow so rapidly that they provide great turnover despite having little mass. FOOD-ENERGY PYRAMID
BIOMASS PYRAMID
At each level of a food chain, energy is lost as heat, so less is available to the next consumer. The diminishing energy at each level can be represented by a pyramid (below) and accounts for the scarcity of top predators.
The biomass pyramid (below) for a system with plankton producers is partly inverted, because the producers have low total mass. Despite this, the rapid reproduction of the plankton keeps the food chain supplied.
top predators
top predators
predators
predators
consumers
consumers
primary producers
Recycling All living things need a supply of chemical nutrients, such as nitrates, phosphates, and silicates, to grow and reproduce. They are taken up by primary producers then passed along the food chain. Although some nutrients are available from seawater, most are derived ultimately from the sea floor. When an organism dies, any parts that are not eaten by other animals gradually sink to the sea floor, where they are broken down by bacteria and other decomposers. Fecal matter also ends up on the seabed and is processed by detritus feeders or decomposers. Eventually, the nutrients are released into the environment in their mineral, nonliving forms. They may then remain at depth, or they may be returned to surface waters by circulating water currents within an ocean basin (see upwelling, opposite). upwelling of nutrients released by bacteria
NUTRIENT CYCLE
phytoplankton absorbs sunlight and use nutrients to grow zooplankton feed on phytoplankton
detritus falls
WARM WATER
HUMAN IMPACT
COLD WATER
fish eats detritus
detritus falls to sea floor
primary producers TOTAL BIOMASS
TOTAL ENERGY
PRIMARY PRODUCERS
PRIMARY CONSUMERS
detritus on sea floor
bacteria process detritus
SECONDARY CONSUMERS
TERTIARY CONSUMERS
QUATERNARY CONSUMERS
phytoplankton krill
Small particles of organic matter, or detritus, are found in the water column. They may be eaten by scavengers or broken down still further by bacteria present in the water. However, many of them rain down on the ocean floor where they decompose, releasing nutrients. The nutrient cycle is completed by upwelling water currents that then carry the nutrients back to the surface where they can be utilized by the phytoplankton.
baleen whales
OVER-HARVESTING These fishermen are harvesting Pacific cod. Cod populations have drastically declined and many important stocks have collapsed because too many are being caught for human consumption before they can reproduce successfully. The imposition of quotas by governments has not solved the problem, although numbers are starting to recover in some areas. COD FISHING
More than 1.5 million tons of cod (Atlantic and Pacific) were caught in 2010 using various methods including trawls and longlines.
birds
protozoans
carnivorous zooplankton
penguins
seals
pelagic fish decomposer bacteria
FOOD WEB squid
OCEAN LIFE
copepods
seaweed
decomposer invertebrate
detritus on sea floor
demersal fish
small toothed whales
killer whales
Many food chains have been combined to form this complex food web, extending from primary producers to quaternary consumers (top predators) for a Southern Ocean ecosystem. Each arrow shows the flow of food energy from prey to predator, grazer, or decomposer. It shows how organisms depend on one another for food. Some animals feed on organisms from several different levels of the food chain, adding to its complexity. Food webs are delicately balanced and easily upset by human interference.
CYCLES OF LIFE AND ENERGY
Productivity
Upwelling
Throughout the world’s oceans, the abundance of marine life varies dramatically. The ocean is more productive in some places and at some times than others. The amount of sunlight is a major influence on productivity and changes with latitude and time of year. The supply of nutrient-rich water from the sea floor and light for photosynthesis is affected by changing water movements and day length, affecting plankton levels. Temperature also affects productivity as it influences the rate of photosynthesis.
The open-ocean surface water can become impoverished, as nutrients are constantly absorbed by phytoplankton and fall with detritus to the sea floor. Nutrient-rich water can be restored to the surface on a large scale by vertical ocean currents in a process called upwelling (see p.60). Near land, coastal upwelling is caused by surface currents, such as the Humboldt Current off South America (see p.58). In the equatorial waters of the Pacific and Atlantic, mid-ocean upwelling occurs when water masses are driven north and south by the trade winds, and cooler, nutrient-rich water rises to take their place. Polar upwelling can happen where winter storms cause intense water movement. When upwelling occurs and there is sufficient sunlight, phytoplankton multiply rapidly to support a vast number of organisms, creating the most productive ocean waters in the world.
CLEAR, TROPICAL OCEAN
Tropical waters are not mixed seasonally, so few nutrients are returned to the surface, and little plankton growth is possible. Here, a solitary turtle cruises in crystal-clear surface waters near Hawaii.
RICH, MURKY TEMPERATE SEA
In coastal and temperate areas, water turbulence circulates nutrient-rich water that supports a variety of algae, such as this kelp forest in the Pacific.
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NUTRIENT-RICH WATERS
Where there is upwelling, large numbers of small fish gather to feed on the plankton. They, in turn, attract larger predators like these copper sharks feeding on sardines off the coast of South Africa.
OCEAN LIFE
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INTRODUCTION TO OCEAN LIFE
Swimming and Drifting MOST OF THE OCEAN’S LIVING SPACE IS NOT ON THE SEABED
but in the water column and out in the open ocean—areas known as the pelagic zone. Salt water provides support, as well as the nutrients that allow many plants and animals to live in the water column without ever going near the seabed. Some animals live at the interface between ocean and air, or alternate between both environments, because it is more energy-efficient. The water surface, water column, and seabed are all interconnected, and many animals move between these habitats.
Plankton The sunlit, surface layers of the ocean are home to many tiny plants and animals (plankton) that drift with the water currents. Phytoplankton consist of bacteria or plantlike chromists (see p.234) that can photosynthesize and make their own food. Along with fixed seaweeds and seagrasses, phytoplankton form the basis of ocean food webs. Zooplankton consists of animals, most of which are very small and feed on the phytoplankton. However, jellyfish can grow to a huge size. Many deep-sea forms have strange shapes and soft bodies that are very delicate. Some zooplankton, such as arrow worms, comb jellies, and copepods, live permanently in the plankton, hunting and grazing (holoplankton), while others are simply the larval and dispersal stages of animals, including crabs, worms, and cnidarians (meroplankton) that will spend part or all of their adult lives on the seabed. Many planktonic organisms have elegant spines, long legs, or feathery appendages that help them PLANKTONIC LARVA float. Tropical zooplankton generally have more The eggs of the of these than their temperate or polar equivalents common shore crab because warm water tends to be less dense and hatch into floating, spiny zoea larva. viscous, and so provides less support.
TEMPORARY PLANKTON
Most temporary zooplankton are the larvae of animals that, as adults, live on the seabed. The common jellyfish, however, has a planktonic adult stage (shown above), and a fixed, asexual, juvenile stage (right).
Nekton Fish and most other free-living marine animals can all swim, even if only for short distances, over the seabed. However, some animals spend their whole lives swimming in the open ocean and are collectively called nekton. This group includes many fish and all whales, dolphins, and other marine mammals, turtles, sea snakes, and cephalopods. There are also some representatives from other groups such as swimming crabs and shrimp. Most nektonic animals are streamlined, TYPICAL NEKTON FEATURE Dusky dolphins are typical of nektonic and there is a remarkable similarity in animals, most of which are vertebrates shape between some dolphins and open(animals with backbones). ocean nektonic fish such as tuna.
OCEAN LIFE
The Ocean–air Interface Some animals live at the interface between air and water, either floating at the surface or alternating between the two environments. Oceanic birds such as albatrosses, petrels, gannets, and tropic birds spend their whole lives out at sea. They eat, sleep, preen, and even mate on the ocean surface. Large rafts of such seabirds are particularly vulnerable to oil spillages. Other diving seabirds, such as terns, alternate between hunting at sea and resting on land. Just as these birds plunge down into the water to catch fish, so some sharks lunge out of the water to catch birds and turtles. Flying fish launch into the air to escape their predators. Some planktonic animals live permanently at the water surface with part of their body projecting into the air. The by-the-wind sailor is a small, colonial cnidarian that is supported by a sail-like float and transported by wind blowing against its vertical sail. Drifting with it on a raft of mucous DRIFTING AT THE INTERFACE bubbles is the violet sea snail, which also feeds The large gas-filled float of the on it. There are even surface-living insects, of Portuguese man-of-war supports the whole colony at the water surface. the genus Halobates, that drift the oceans.
FLYING AND DIVING
The brown pelican is one of several species that dive or dip down from the air into the water to catch fish. It uses its capacious beak as a scoop.
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FLOATING COMMUNITY
Ocean sunfish often drift at the ocean’s surface. They will investigate floating rafts of seaweed and logs for potential food such as small fish and crustaceans.
Drifting Homes
LURING FISH Fish-attracting devices (FADs) have increased catches in many areas by making fish stocks easier to exploit. However, they make no contribution to biological productivity because they simply gather fish together, and so may contribute to overexploitation. Artificial reefs also attract fish, but provide safe breeding sites, too.
FISH-ATTRACTING DEVICE
Even simple FADs, such as this floating buoy in Hawaii, will attract fish. Juvenile jacks and endemic Hawaiian damselfish can be seen sheltering under this one.
OCEAN LIFE
Many pelagic fish species are attracted to floating objects that provide shelter from predators, currents, and even sunlight. Floating logs and seaweed also provide a meeting point. Fishermen have exploited this tendency by using fish-attracting devices (FADs, see panel, right) to concentrate fish in one area. These vary from simple rafts with hanging coconut palm leaves to complex technological devices. Mini-ecosystems often develop on and around large drifting logs. Seaweeds and goose barnacles settle, providing shelter and food for crabs, worms, and fish. Shipworms bore into the wood, and their tunnels provide further refuge. Occasionally reptiles, insects, and plant seeds SARGASSO HAVEN survive and drift on logs, Floating Sargassum seaweed and may eventually be provides a safe haven for the washed ashore to colonize sargassumfish. More than new places, including new 50 animal species have been recorded in this habitat. volcanic islands.
HUMAN IMPACT
216
INTRODUCTION TO OCEAN LIFE
Bottom-living ANIMALS LIVING ON THE OCEAN FLOOR
or within its sand and mud, either moving over it or firmly attached, are called benthic animals. On land, plants provide a structural habitat within which animals live. In the ocean, this is rarely the case, except in shallow, sunlit areas dominated by kelp, seaweeds, or seagrasses. Instead, wherever areas of hard sea bed provide a stable foundation, a growth of benthic animals develops, fixed to the sea bed and often resembling plants. A sea bed of shifting sediments is no place for fixed animals. Here, a community of burrowers develops instead. BENEATH THE SEAWEED
Below the seaweed-dominated zone around northern European coasts, on sea beds too deep and dark for photosynthesis, dead man’s fingers, sponges, and tube worms typically grow attached to subtidal rocks.
Fixed Animals Many benthic animals such as sponges, sea squirts, corals, and hydroids spend their entire adult lives fixed to the sea bed, unable to move around. On land, animals must move around in search of food, whether they are grazers, predators, or scavengers. In the ocean, water currents carry an abundant supply of food in the form of plankton and floating dead organic matter. Fixed animals can take advantage of this by simply catching, trapping, or filtering their food directly from the water, without having to move from place to place. When it is time to reproduce, they simply shed eggs and sperm into the water, where the eggs are fertilized and grow into planktonic larvae. Sometimes, they retain their larvae or eggs, and release them REEF-FORMING TUBE WORM In some Scottish sea lochs, the only when the young are well developed. Water currents distribute the offspring to chalky cases of tube worms form substantial reefs. new areas, where they can settle and grow.
Mobile Animals
OCEAN LIFE
Dense growths of seaweeds or fixed animals provide shelter and food for many mobile animals. Grazers, such as sea urchins, crawl through the undergrowth, eating both seaweeds and fixed animals. Meanwhile, crabs, lobsters, and starfish scramble and swim around, hunting and scavenging for food. Sea slugs are specialist predators, each species feeding on one, or a few, types of bryozoans, hydroids, or sponges. Sea slugs therefore live in close association with their prey and rarely stray far. Kelp holdfasts provide a safe haven for small, mobile animals such as worms.
FISH IN DISGUISE
Scorpionfish live on the seabed among the seaweeds and fixed animals. Their intricate skin-flaps blend in with this habitat.
SEABED IN THE SUN
Seaweeds anchor in the tidal zone of rocky shores and on rocky reefs, such as this one in the Canary Islands. On sunlit, temperate sea beds, it is seaweeds that provide the community structure.
BOTTOM-LIVING
217
Burrowing and Boring Much of the seafloor is covered in soft sediments, such as sand and mud. Living on the surface of the sediment is both difficult and dangerous, and most animals burrow below or build tubes in which to live and hide. Bivalve molluscs and segmented worms cope especially well in this habitat, and many different species can be found in sediments all over the world. Safe under the sediment surface, a bivalve draws in oxygen-rich water and plankton through one of its two long siphons, expelling waste through the other. It never has to come out to feed or breathe. Piddocks and shipworms bore into rocks and wood, then use their siphons in a similar way. Sediment is not a completely safe home—predatory moon snails dig through sand and bore into bivalve shells, eating the contents. Ragworms are also active predators, hunting through the sediment for other worms and crustaceans. Some worms build flexible tubes from sand grains, their own secretions, or both. The tubes stick out of the sand, and they feed by extending feathery or sticky tentacles from the tube to catch plankton. If danger threatens, they can withdraw rapidly. A similar strategy is adopted by tube anemones and sea pens.
REPLACING SIPHONS
BORING INTO ROCK
The siphon tops of buried bivalve mollusks are sometimes nipped off by flatfish but can regrow.
The boring sponge uses chemicals to dissolve tunnels in calcareous shells and rocks, creating a living space for itself.
FIXED TO THE BOTTOM
Christmas-tree worms live attached to the bottom in hard tubes that they cement into coral reefs. They feed by filtering plankton from the water, using their beautiful double spiral of tentacles.
Symbiosis Bottom-living is a challenge for marine organisms. A safe crevice on a coral reef, for instance, is valuable, but fiercely fought over. The solution to finding a home is often to enter an intimate relationship with a different organism—a situation called symbiosis. When only one partner benefits, the relationship is called commensal, and often involves one animal providing a home for the other. MUTUAL RELATIONSHIP Small pea crabs live inside mussels, The Banded Coral Shrimp earns its place gaining shelter and food, while the in the moray eel’s well-defended crevice by cleaning the teeth of its host. mussel merely tolerates their presence. Symbiosis in which both partners benefit is called mutualism. Many tropical gobies live in such relationships with blind or nearly-blind shrimp. The shrimp digs and maintains a sandy burrow that accommodates both, while its sharp-eyed partner goby acts as a lookout. Some anemones adhere to the shells of hermit crabs, gaining from the crab’s mobility and access to its food scraps. The crab is protected, in return, by the anemone’s stinging tentacles. The third type of symbiosis is parasitism, in which one partner, the host, is harmed. The crustacean Sacculina spreads funguslike strands through its host crab’s body to extract nutrients, weakening or killing the crab.
Large reef anemones often provide a haven for clownfish and tiny cleaner shrimp. The anemone benefits from the housekeeping activities of its guests.
OCEAN LIFE
A HOME IN EXCHANGE FOR CLEANING
218
INTRODUCTION TO OCEAN LIFE
Zones of Ocean Life
HUMAN IMPACT
NO PART OF THE OCEAN IS DEVOID
The northern and southern geographical limits of many shallow-water marine species are dictated by water temperature. Most species breed and disperse only within certain temperature limits. Climate change is slowly raising water temperatures and in the Northern Hemisphere, records have shown that some warmwater species are extending their ranges farther north. Similarly, some cold-water species may be expected to retreat farther north.
of organisms, from polar seas to the tropics and from coasts and the seashore to the deepest depths. The seabed and the water column above it both support a huge variety of life. However, marine organisms are distributed unevenly both horizontally and vertically. As on land, climate (mainly temperature) and food play a large part in determining distributions and biodiversity. In the harsh environment at the poles, there is less coastal life than in the warm tropics, but beneath the surface, Antarctic seas support rich marine communities. Although there is life at every depth, most creatures can only survive within particular depth zones at particular pressures, temperatures, and light regimens.
Geographical Zones Seawater temperatures are much more stable than those on land because water loses and gains heat more slowly than does air. However, the distribution of marine coastal and continental shelf communities still follows a global pattern, with distinct polar, temperate, and tropical ecosystems. Coastal salt marsh in temperate parts is replaced in the tropics by mangroves. Kelp forests only grow in cool waters but extend into the tropics in places where cold water upwells from the deep, such as off the coast of Oman on the Arabian peninsula. Planktonic species and bottom-living species with planktonic larvae might be expected to occur anywhere that ocean currents take them. However, a boundary between water masses with different physical characteristics may present as effective a barrier in an ocean as mountains do on land. Below a certain depth, there are fewer such barriers, and conditions are stable and similar worldwide, so deep-sea animals often have very wide distributions. KEY
CLIMATIC ZONES
The shape and tilt of our planet results in differences in the amount of solar radiation reaching land and ocean at different latitudes. This produces large-scale climatic zones that ring Earth.
equatorial
temperate
tropical
subpolar
subtropical
polar
OCEAN LIFE
Endemic Species Some marine organisms, especially pelagic species, have a wide MALDIVES ANEMONEFISH global distribution, since there are few barriers to their dispersal. This endemic fish is not a Others live in restricted geographical ranges and are said to be strong swimmer. It does not planktonic larvae and endemic to a particular sea, island, or country. The most remote have lives only in the Maldives patches of habitat, such as small oceanic islands, tend to have and Sri Lanka in the Indian the most endemic species. This is Ocean. Its host anemone has a wider distribution, because animals in their dispersive because its larvae disperse stages, such as eggs and larvae, may on ocean currents. survive only for short periods and so never reach distant shores. The Red Sea holds many endemic fish species. It is connected to the Indian Ocean only by a narrow channel and so is effectively isolated. Endemic fish are often those that cannot or do not swim GALAPAGOS PENGUIN far. Anemonefish, for example, lay This penguin species lives only their eggs on rocks under their around the Galápagos islands. The cold, upwelling Cromwell anemones and the young search for Current keeps them cool in spite new anemones on the same reef. of the tropical climate. They Flightless marine birds such as are isolated by the surrounding warm waters, so cannot disperse penguins are likewise restricted in beyond their home islands. their ability to colonize new areas.
SHIFTING ZONES
TROPICAL INVADER
Warm-water triggerfish stray as far north as southern Britain and have now begun to breed there. With continued ocean warming, they may become a native species.
ZONES OF OCEAN LIFE
219
Depth Zones As depth increases, so does pressure, while light, temperature, and food supply decrease. These changes impose limits on the types of marine organisms that can survive and prosper at different depths. The areas on and over continental shelves around the world are rich in life as they are well-supplied with nutrients from river discharge and stirred-up sediments. Shoaling fish, such as herring, feed on plankton sustained by the nutrients. Most commercial fisheries are over continental shelves. Below the continental shelf, no phytoplankton or seaweeds grow. Pelagic animals either eat each other or make daily feeding migrations into the upper layers. Rocky areas support a diverse fauna including coldwater coral reefs, sponge reefs, and hydrothermal vent communities. Fine sediments cover the immense, flat abyssal plains at 0 ft the foot of the continental slope. seaweeds While microorganisms abound, 160 ft sponge (50 m) large animals are relatively scarce. starfish
330 ft (100 m)
phytoplankton
zooplankton
500 ft (150 m) 650 ft (200 m)
SEABED IN THE SUNLIT ZONE
whale shark
Portuguese man-of-war
660 ft (200 m) SUNLIT ZONE On seabed, high biodiversity— seaweeds, corals, sessile animals; in water, rich plankton, abundant fish, cetaceans
mackerel salp
shark
TWILIGHT ZONE On seabed, crinoids, sponges, sea fans, sea pens, 6,500 ft sea cucumbers, Greenland (2,000 m) shark; in water, zooplankton, squid, shrimp, predators— sperm whale, silvery fish with large eyes, such as hatchet fish and lanternfish DARK ZONE On seabed, similar to twilight zone; in water, mostly small, darkcolored fish with large mouths and stomachs, gulper eels, rattails, anglerfish, red shrimp, deep-sea jellies
Seal breathing holes in sea ice create oases on the seabed beneath, where benthic organisms enjoy the benefits of a greater supply of light and nutrients.
squid
hatchetfish
Environmental conditions change gradually as depth increases, but zones can be recognized based on both physical and biological parameters. The types of marine life in each zone are shown here.
comb jelly
crinoid sponge
deep-sea anglerfish
9,800 ft (3,000 m)
VERTICAL LIFE ZONES
OASIS BENEATH THE ICE
tuna jellyfish
3,300 ft (1,000 m)
black swallower
hagfish ABYSSAL ZONE On seabed, few large animals, rattails, hagfish, and sea cucumbers, very diverse protists, nematode worms, bacteria; in water, some deep-sea fish
13,100 ft (4,000 m)
16,400 ft (5,000 m) HADAL ZONE Little-known region, but some large organisms found in deepest depths; deepest fish caught at 26,200 ft (8,000 m) 19,700 ft (6,000 m)
Ocean Deserts
OCEAN LIFE
cusk eel Some areas of ocean are similar to deserts on land and support few species. Clear, blue surface water over the deep oceans often supports only small amounts of plankton, because it is very poor in the nutrients and minerals needed by phytoplankton to grow. This is especially the case in areas where there are few storms to stir the water and bring nutrients up from deep water. The nutrient iron can be a limiting factor, and experiments in which areas were seeded with iron have shown greatly increased phytoplankton production. The ocean floor in abyssal depths can support only a few large animals BARREN POLAR SHORE and was once considered to be a virtual desert. This polar shore in Greenland However, recent work on deep-sea sediments has supports little life due to the grinding action of winter ice, though shown the opposite. If all the bacteria and tiny animals living between the sediment particles are counted, below the reach of the ice, rich communities may develop. then this habitat is as diverse as a tropical rainforest.
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INTRODUCTION TO OCEAN LIFE
Ocean Migrations
DISCOVERY
FEEDING AND BREEDING ARE THE MAIN REASONS
Until recently, any journey made by a marine animal, such as the leatherback turtle shown below, was poorly understood because tracking devices used on land were inappropriate for use in water. This changed when satellitetracking devices became available. Attaching one to a turtle does not impede or harm it in any way but it can still pose problems.Yet turtles are threatened in the wild, so knowing where a female goes after laying her eggs is vital to conservation work.
TRACKING
that animals migrate. They move from one place to another, often at the same time of day or year, and usually follow the same, well-defined routes. Migratory species include many of the larger marine animals, such as whales and turtles, but smaller creatures, such as squid and plankton, also make spectacular journeys in order to survive and reproduce. Animal migration in the oceans is more complicated than on land because animals can move both horizontally and vertically through the water column.
Types of Migration The driving force behind any animal migration is survival. Individuals must eat to live, and some will travel long distances to find food. Such journeys often coincide with peak production times of plankton and other food sources in particular places, such as sites of seasonal upwellings. A shorter, more regular feeding migration is made daily by plankton and active swimmers such as squid (see p.221). A species’ survival depends on reproductive success. Gathering together and breeding in a few places at the same time optimizes conditions for offspring survival. Breeding grounds where food is abundant and conditions favorable are used repeatedly, with individuals often returning to their birthplace to breed. Some shore animals also migrate up and down the beach, following the outgoing tide to feed and avoiding immersion by returning before the tide turns.
T
NURSERY AREA
SPAWNING SITE
migration as plankton
MARINE MIGRATORY CYCLE
Some marine organisms migrate to a specific spawning site to release their eggs. The eggs hatch into larvae that join the plankton and drift in the currents to another nursery area, where they feed and mature before joining the adult population.
I OCEAN
OCEAN
A tracking device is being attached to this leatherback turtle on Juno Beach, Florida. In case an opportunity does not arise to remove it manually, parts of the harness are designed to gradually disintegrate.
spawning migration
OCEAN
C
PACIFIC
TRACKING TURTLES
migration of young adults
PACIFIC
N
migration route
ARCTIC TERN MIGRATION
return of adults after spawning
AT L A
MIGRATING TERN
ADULT POPULATION
INDIAN OCEAN
This small bird flies from the Antarctic to the Arctic to breed and then returns south, a round trip of nearly 22,000 miles (35,500 km). Terns spend 90 days at the nesting grounds each year. The rest of the time is spent mostly on the wing. summer distribution
winter distribution
LOBSTER MIGRATION
Caribbean spiny lobsters migrate in single file across the sea floor in winter, seeking warmer water, and return to shallow water in summer.
Migrating between Salt and Fresh Water Although some marine species can cope with a great range of salinity and temperature, only a few move between fresh and salt water at particular stages of their lives. Some, such as salmon, start and finish their lives in fresh water and spend the rest of their time in the ocean. Such fish are described as anadromous. Eels, on the other hand, start and finish their lives in the sea, but spend 10 to 14 years in fresh water while maturing. Fish such as these are termed catadromous. At maturity, both of these fish return to their birthplace to breed, after which they die. Changing from fresh to salt water or vice versa would be fatal to most fish, but various physiological adaptations, including the way their kidneys function, allow both anadromous and catadromous fish to make the transition without experiencing any ill effects.
OCEAN LIFE
SALMON RETURNS TO FRESHWATER SPAWNING GROUNDS
At three to five years of age, a coho salmon is ready to return to the river where it was born to spawn. Some mature at only two years, returning to their home river as “jacks.”
1
In migrating upstream to its spawning grounds, the salmon may swim 2,175 miles (3,500 km) against the water current, negotiating several waterfalls and rapids.
2
As soon as the female has deposited her eggs in the gravel on the riverbed, the male swims over them and releases his sperm, optimizing the chance of fertilization.
3
Newly hatched salmon live among the gravel until they absorb their yolk sacs and become fry. They then begin their journey downstream to the sea.
4
221
HORIZONTAL AND VERTICAL TRAVEL
Some animals migrate horizontally and vertically. Here, longfin squid migrate to spawn in May. They also move up and down the water column each day to feed.
Navigation While satellite tracking provides information on migration routes and confirms that many individuals travel the same path, how animals navigate over vast distances is still poorly understood. Salmon return to their spawning grounds by smelling the unique chemical composition of the water, but they first have to get close enough to pick up this scent. Some aquatic species may use water currents to guide them, while others use the Earth’s magnetic field to navigate. Animal migration is amazingly accurate, in terms of both direction and timing. Oceanic birds, turtles, and mammals can also navigate using the sun, the stars, and familiar landmarks.
DAY
Vertical Migrations
NIGHT
phytoplankton
phytoplankton
mackerel
jellyfish lanternfish copepods
mackerel
shark squid
660 ft (200 m)
BELUGA WHALES MIGRATING
copepods
Belugas, like these in Lancaster Sound, Canada, live in the Arctic and subarctic, but some migrate to warmer waters in summer.
squid
In temperate and tropical regions, zooplankton migrates to the ocean surface at night and then moves down again during the hours of daylight. In a single day, this vertical movement may range from 1,310 to 3,300 ft (400 to 1,000 m), depending on the size and type of animal involved. In polar regions, where darkness lasts for several months, zooplankton migrates up and down on a seasonal basis, being at the surface during summer and at depth in winter. It is thought that zooplankton rises to feed on the phytoplankton that lives in the surface waters, but then retreats to depth for safety, or possibly because it expends less energy in cooler water. Maturing planktonic larvae of animals such as crabs will eventually migrate to the sea floor and become benthic.
jellyfish
EARTH’S GREATEST MASS MIGRATION
lanternfish squid
3,300 ft (1,000 m)
OCEAN LIFE
shark
During the day, when many animals remain in the depths, out of sight of predators, the phytoplankton utilizes the Sun’s energy to produce food. At night, the biomass of the surface waters (the sunlit zone) increases by as much as 30 percent, as zooplankton comes up to feed on phytoplankton and is, in turn, eaten by various fish and other animals. This regular movement of animals up and down the water column is the greatest mass migration on Earth.
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INTRODUCTION TO OCEAN LIFE
Living Down Deep
THE DEEP-SEA ENVIRONMENT APPEARS INHOSPITABLE—cold, dark, and
with little food. However, it is remarkably stable: temperatures remain between 35 and 39ºF (2 and 4ºC) year-round, salinity is constant, and the perpetual darkness is overcome by novel communication methods (see pp.224–225). Although deep-sea pressures are immense, most marine animals are unaffected, since they have no air spaces, while animals living below about 5,000 ft (1,500 m) show subtle adaptations. Species diversity of large animals decreases with depth, but there is a huge diversity of small organisms living within deep-sea sediments.
OCEAN LIFE
Pressure Problems
DEEP-SEA ADAPTATIONS
Anglerfish have a lightweight skeleton and muscles for neutral buoyancy. This specimen’s muscles have been “cleared” to show the bone, which is stained red.
Deep-ocean animals experience huge pressures, but problems arise only in gas-filled organs such as the lungs of diving mammals and the swim bladders of fish. Sperm whales, Weddell seals, and elephant seals all dive to depths where their lungs are compressed, but their flexible rib cages allow this. While underwater, they use oxygen stored in blood and muscles. Deep-sea fish can cover a large vertical range because pressure changes at depth are proportionately less, per foot, than near the surface, so the pressure or size of their swim bladders does not change radically. In oceanic trenches, the pressure is so great that it affects the operation of biological molecules, such as proteins. Pressure-loving bacteria in this habitat have specialized SPERM WHALE proteins—they cannot Sperm whales can dive to at least 3,300 ft (1,000 m), where grow or reproduce when brought to the pressure is 100 times greater than at the surface. the surface.
LIVING DOWN DEEP
Finding Food
mouth surrounded by modified tube feet
The major problem of deep-sea living is finding enough food. With the exception of communities based around hydrothermal vents and cold seeps (see pp.188–89), animals living in the deep ocean and on the deep-ocean floor are ultimately reliant on food production in the sunlit layer, thousands of feet above. In the depths, it is too dark for plant plankton to live and to provide food. Sometimes, large mammal or fish carcasses reach the sea bed, but most food arrives as tiny food fragments, slowly sinking from above. Much is eaten before it reaches the sea floor, but much is also added in the form of skins, shed from mid-water crustaceans and salps. Bacteria grow on such material, helping it to clump together and so fall more rapidly.
SEABED CONSUMER
The fangtooth lives at midwater depths of about 1,600–6,500 ft (500–2,000 m). Food is scarce, so its large mouth and sharp teeth help it to catch all available prey.
DISCOVERY
OBSERVING DEEP-SEA LIFE Before the advent of modern research submersibles, few biologists had the opportunity to see deep-sea animals alive and in the wild. Dredged and netted specimens are often damaged, and little can be learned from them about the animal’s way of life. Modern submersibles have an excellent field of view, are equipped with sophisticated cameras and collecting equipment, and can operate to depths of 3,300 ft (1,000 m) or even 20,000 ft (6,000 m).
tube foot, used to move across seafloor
MIDWATER FEEDER
223
Sea cucumbers vacuum up organic remains from the sea floor. At high latitudes, more food rains down in spring, following surface phytoplankton blooms; these rains may trigger sea cucumbers to reproduce.
Scavenging Giants Many deep-sea animals are smaller than their relatives in shallow water. This is an evolutionary response to the difficulties of finding food in the deep ocean. However, some scavengers survive by growing much larger than their shallow-water counterparts. For example, amphipod and isopod crustaceans that measure only about ½ in (1 cm) long are common in shallow water, where they scavenge on rotting seaweed and other debris. Carrion in the deep sea is sparse, but it comes in big, tough lumps such as whale carcasses. Some deep-sea amphipods grow to a length of 4–6 in (10–15 cm), more than ten times larger than shallow-water species, and so are able to tackle such a bonanza. In the low temperature of the deep ocean, these animals move and grow slowly and reproduce infrequently, but live much longer than their shallow-water counterparts. Sea urchins, hydroids, seapens, and other animals also have giant deep-sea forms. Similar giants are found in cold Antarctic waters.
A WINDOW ON DEEP-SEA LIFE
Deep Rover is a two-person submersible capable of diving to 3,300 ft (1,000 m), launched from a semi-submersible platform. The occupants can see all the way around through the acrylic hull.
Staying Aloft Huge areas of the deep-sea floor are covered in soft sediments many yards thick, called oozes (see p.181). Seabed animals need ways of staying above these sediments so that they can feed and breathe effectively. Many sedentary filter-feeding animals, such as sea lilies, sea pens, and some sponges, have long stalks, enabling them to keep their feeding structures above the sediment. Some sea cucumbers have developed stiltlike tube feet that help them walk over the sediment surface, instead of having to plow through it. Similarly, the tripodfish props itself up on its fin tips. One species of sea cucumber, Paelopatides grisea, has an unusually flattened shape that allows it to lift itself off the sea bed with slow undulations of its body. SEA LILIES ANCHORED IN THE OOZE
To catch food, sea lilies reach up into the current on stalks up to 2ft (60cm) high. The stalk extends deep into the sediment to provide an anchor.
DEEP-SEA GIANT
The widespread deep-sea scavenger amphipod Eurythenes grows to over 3 in (8 cm).
OCEAN LIFE
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INTRODUCTION TO OCEAN LIFE
Bioluminescence BIOLUMINESCENCE IS A COLD LIGHT
produced by living organisms. On land, only a few nocturnal animals, such as fireflies, produce light, but in the ocean, thousands of species do so. Deep-water fish and squid use bioluminescence extensively, but there are many other light producers, such as species of bacteria, dinoflagellates, sea pens, jellyfish, mollusks, crustaceans, and echinoderms. Evidence suggests that marine organisms use bioluminescence for defense (as camouflage or distraction), for finding and luring prey, and for recognizing and signaling to potential mates.
Light Production
pigment cup
Many bioluminescent marine organisms use their light in communication. This bristlemouth fish can signal to its own kind with its specific photophore pattern.
light source lens
rays Bioluminescence is produced by a chemical into reaction in special cells known as photocytes, focused beam and carried away usually contained within light organs called from source photophores. A light-producing compound called luciferin is oxidized with the help of an LENS enzyme called luciferase, releasing energy in the form of a cold light. Most bioluminescent light is pigment light cup source blue-green, but some animals can produce green, yellow, or, more rarely, red light. A range of light-producing structures is found in different animals. The hydroid Obelia has single photocytes scattered in its tissues, while certain fish and squid have complex photophores with lenses and light filters. Some animals, including flashlight and eyelight pipe LIGHT PIPE fish, some anglerfish, ponyfish, and some squid, adopt a different strategy. pigment TYPES light They culture symbiotic, PHOTOPHORE cup source Photophores often feature a bioluminescent bacteria pigment cup and a lens that directs in special organs. The the light into a parallel beam. With a light pipe, light can be bacteria produce their filter allows light and are, in return, channeled from the photophore, deep-red which might be buried in the only pigment fed nutrients by their animal’s body. Color filters in deep-red filter light host and given a safe front of the light source fine-tune COLOR FILTER to pass place in which to live. the color of the emitted light.
HUNTING WITH A SPOTLIGHT
The dragonfish produces a beam of red light, from a photophore beneath its eye, to spotlight its prey. Red light is invisible to most deep-sea animals.
Light Disguise
body covered with tiny, flashing photophores
Animals using bioluminescence to attract prey, or to signal to each other, risk alerting their own predators to their presence. However, lights can also be used for camouflage. Hatchetfish live at depths where some surface light is still dimly visible. To keep their silhouette from being seen from below, they manipulate the light they emit from photophores along their belly, to mimic the intensity and direction of the light coming from above. Bioluminescence is also used to MANIPULATING LIGHT confuse potential predators. Flashlight fish The silvery, vertical flanks of turn their cheek lights on and off. Some hatchetfish reflect downwelling squid, shrimp, and worms eject luminous light, and their photophores shine secretions or break off luminous body parts downward, camouflaging their silhouette from below. that act as decoys, while they escape.
OCEAN LIFE
USING LIGHT TO COMMUNICATE
LUMINOUS SMOKESCREEN
organs producing downward-directed beams of light
A firefly squid presents a predator with a myriad of confusing pinprick lights emitted from its body. It can also secrete a cloud of luminous particles into the water to act as a smokescreen, allowing it to escape.
squid’s ink is bioluminescent
BIOLUMINESCENCE
bioluminescent organ produces light and directs it downward; the light merges with light downwelling from the sky and conceals the animal from predators below
225
light organs form a distinctive pattern recognized by other bristlemouths
Predators In the unlit regions of the deep ocean, many hunters try to attract prey, rather than go in search of it. After all, hunting light-producing bacteria cause by sight and chasing prey is difficult lure to glow where the only available light is from bioluminescence. An obvious way of attracting prey is to use a luminous lure, and anglerfish are especially good at this. Anglerfish in the genus Linophryne have a head lure, like a fishing rod, lit by luminous bacteria, and a chin barbel with tiny photophores that produce their own light. Midwater fish often have thin skeletons and weak muscles to improve their buoyancy, so luring prey is an energy-efficient way for them to hunt. Stauroteuthis syrtensis, an `unusual deep-sea octopus with glowing suckers, sets a deadly trap. Its eight tentacles are connected into a web, and GLOWING JELLYFISH its modified suckers, which have lost the ability to LUMINOUS LURE The mauve stinger glows grasp, are bioluminescent. Although this species has Fish are attracted to the luminous lure of with bioluminescence when never been seen hunting, its prey (which are primarily deep-sea anglerfish and are quickly snapped it is disturbed by waves, and copepods) is probably lured toward the raised, up. Most anglerfish are brown or black so can also produce a luminous that they do not light themselves up. mucus if it is touched. light-emitting arms, and then enfolded and eaten.
Phosphorescence
Dinoflagellates are tiny, single-celled organisms that emit bright flashes of light when disturbed. In large numbers, they produce “phosphorescent” seas.
OCEAN LIFE
On a still, warm night, especially in the tropics, moving boats leave a glittering trail of light in their wake and divers can create swirling pinpricks of light by simply moving around. This phenomenon is caused by bioluminescent plankton, mostly dinoflagellates. Their light is often informally called phophorescence, because it is emitted when they are disturbed, but decays after a few seconds. Biological phosphorescence is thought to be an anti-predation device. When dinoflagellates are attacked by planktonic copepods, they flash. This alerts nearby shrimp and fish to the copepods’ presence, and the copepods themselves may then become prey. Some dinoflagellates, such as Gonyaulax polyedra, only produce light at night, so they do not waste energy on light production when it cannot be seen. Deep-sea jellyfish may use a similar antipredator strategy. The jellyfish light up only when disturbed by vibrations, which indicate an approaching predator. Often, a series of erratic flashes travels over the entire body surface. Such lights may serve to distract the predator.
BIOLUMINESCENT PLANKTON
226
INTRODUCTION TO OCEAN LIFE
The History of Ocean Life LIFE HAS BEEN PRESENT IN THE OCEANS
for over 3,500 million years. The great diversity of today’s marine life represents only a minute proportion of all species that have ever lived. Evidence of early life is hard to find, but it is seen in a few ancient sedimentary rocks. The fossil record has many gaps, but it is the only record of what past life looked like. Fortunately, many marine organisms have shells, carapaces, or other hard body parts, such as bones and teeth. They are more likely to be preserved than entirely soft-bodied creatures, although in exceptional circumstances these have also been fossilized. Using fossils, and information from the sediments in which they are preserved, scientists can reconstruct the history of marine life.
3,800–2,200 Million Years Ago The Origin of Life
EARLY MICROFOSSILS
This micrograph of a section of chert (a form of silica) from the Gunflint Formation, Canada, includes 2,000 million-year-old microfossil remains. These micro-fossils contain the oldest and best-preserved fossil cells known.
When arth formed, it was totally unsuitable for life. The atmosphere changed, however, and the oceans formed and cooled (see pp.42–43), so that by 3,800 million years ago, conditions allowed biochemical reactions to take place. It is thought that simple, water-soluble organic compounds called amino acids accumulated in the water, eventually forming chains and creating proteins. These combined with other organic compounds, including selfreplicating DNA, to form the first living cells. Earth’s atmosphere was further developed by mats of algae and cyanobacteria called stromatolites, whose fossil record stretches from over 3,500 million years ago to the present day. Stromatolites could perform photosynthesis, and their growth eventually flooded the atmosphere with oxygen. Cyanobacteria are single-celled organisms with DNA but no nucleus or complex cell organelles. It was not until 2,200 million years ago that cells with nuclei and complex organelles (eukaryote cells) appeared.
620–542 MYA Precambrian Life Ancient life, though soft-bodied, fossilizes under certain conditions, offering rare glimpses of early multicellular life. About 620 million years ago, a community of soft-bodied animals known as the Ediacaran fauna left their body impressions and trackways in a shallow sea bed. The sea bed now forms the sandstone of the Ediacaran Hills in Australia, where the fauna was discovered in the 1940s. The ancient sea was inhabited by strange, multicellular animals. Some resembled worms and jellyfish, but others were thin, flat, and unfamiliar, making it difficult to know if they are related to existing animals or a separate, extinct, evolutionary line. These animals are the only link between the single-celled organisms that preceded them and the rapid diversification of life that followed. Ediacaran fauna are also found in Namibia, Sweden, Eastern Europe, Canada, and the UK. EDIACARAN FOSSILS
OCEAN LIFE
These are typical examples of Ediacaran fossils preserved as impressions in rock. Mawsonite (left) is believed to be a complex animal burrow; Spriggina (below) may be an arthropod, or a new life-form.
PEOPLE
A.I. OPARIN In 1924, Russian biochemist Aleksandr Oparin (1894-1980) theorized that life originated in the oceans. He suggested that simple substances in ancient seas harnessed sunlight to generate organic compounds found in cells. These compounds eventually evolved into a living cell.
THE HISTORY OF OCEAN LIFE
227
550–530 MYA Cambrian Explosion
FIRST REEFS
The Cambrian reefs were built by extinct sponges called archaeocyathids. They resembled tube sponges (above), having a similar shape and a calcareous skeleton.
Over the course of 20 million years, around the start of the Cambrian Period, many life-forms made a sudden appearance. Indeed, most of today’s major animal groups (phyla) abruptly appear in the fossil record. The Cambrian Explosion of evolution may have been caused by the creation of new ecological niches as the coastline increased, due to the breakup of the Rodinia supercontinent. Further niches arose as a rise in sea level produced large expanses of warm, shallow water. The Cambrian seas were dominated by arthropods, chiefly trilobites, but there were also foraminiferans, sponges, corals, bivalves, and brachiopods. All readily fossilize, as they each have some sort of mineralized “skeleton.” ARTHROPOD TRAILBLAZERS
Trilobites evolved a multitude of different body forms and remained a ubiquitous arthropod group for the next 100 million years. They became extinct during the Permian Period.
BRACHIOPODS
Brachiopods may resemble bivalve molluscs, but they are unrelated life-forms and were among the first animals to appear in the Cambrian Period. Over 3,000 genera have been described. Only about 400 species survive today.
418–354 MYA The Age of Fish LIVING MARINE STROMATOLITES
Built by Earth’s oldest type of organism, stromatolites are now found in only a few places such as here, in the hyper-saline water of Hamelin Pool, Australia.
The earliest vertebrate fossils known are jawless fish that lived some 468 million years ago. Jawed fish appeared in the Silurian Period, following the development of massive coral and sponge reefs that provided them with a multitude of habitats in which to diversify. The now-extinct acanthodians, with their prominent spines on the leading edges of their fins, were among the earliest of these. Having hinged jaws allowed fish to feed more efficiently, and paired fins gave them the speed and manoeuvrability to hunt. The following Devonian Period (418–354 million years ago) saw an evolutionary radiation that could be called the “Age of Fish.” Armored fish called placoderms dominated Devonian seas, some reaching lengths of 20 ft (6 m). Ray-finned fish, sharks, and lobe-finned fish also appeared at this time and have survived to the present day, although marine lobe-finned fish are known only from the Coelacanth. Lobe-finned fish are important in the fossil record because one group gave rise to early tetrapods (limbed vertebrates).
EVOLUTIONARY INSIGHT EARLY JAWLESS FISH
Jawless fish first evolved in the ocean, later spreading into brackish and freshwater habitats. The bony head shield and dorsally situated eyes of this Cephalaspis suggest it is a bottom-dweller.
This lobe-finned fish, Tiktaalik roseae, has gills and scales like a fish, but has tetrapodlike limbs and joints. This “missing link” helps to reveal how animals moved from the oceans onto land.
Mimia, a small, ray-finned fish
Cladoselachii, one of the few sharks in the seas at this time
The Devonian reef fauna (right) from Gogo, Australia, is typical of the time. It is dominated by a wide variety of armored placoderms, but ray- and lobe-finned fish, and a shark, have also been found there.
Rolfosteus, a long-snouted placoderm with crushing toothplates
Bothriolepis, a bottom-dwelling placoderm
phyllocarid, a relatively common shrimp-like crustacean
Nautiloid, a primitive marine shelled cephalopod
rugose coral; common in Devonian seas, now extinct tabulate coral, now extinct
OCEAN LIFE
DEVONIAN COMMUNITY
Eastmanosteus, a large placoderm and active predator
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INTRODUCTION TO OCEAN LIFE
252–65 MYA Giant Marine Reptiles
PEOPLE
MARY ANNING
During Triassic, Jurassic, and Cretaceous times, evolution of reptiles, similar to that of the dinosaurs on land, occurred in the oceans. Between 252 and 227 million years ago, three groups appeared—turtlelike placodonts, lizardlike nothosaurs, and dolphinlike ichthyosaurs. Of these, only ichthyosaurs survived until the Jurassic. The Jurassic oceans teemed with life. Modern fish groups were well represented, as were ammonites, mollusks, squid, and modern corals. A variety of ichthyosaurs evolved, some giant forms reaching 30 ft (9 m) in length, but they soon died out and were replaced by modern sharks. The gap left by the extinction of the placodonts and nothosaurs was filled by long-necked plesiosaurs. Those with a short body and tail and a small head lived in shallow water, while larger forms, called pliosaurs, probably lived in deep water. It is also likely that some of the flying reptiles, called pterosaurs, lived on coastal cliffs and survived by eating fish caught at the water’s surface. During the Cretaceous Period, reptiles remained the largest marine carnivores, (plesiosaurs now coexisting with mosasaurs, distant relatives of monitor lizards,) but none survived the mass extinction that occurred 65 million years ago.
Lyme Regis in Dorset, UK, is famous for its Jurassic fossils, and is where Mary Anning (1799-1847) found and collected her nowfamous ichthyosaur and plesiosaur skeletons. She was one of the first professional fossil collectors. FOSSILIZED ICHTHYOSAUR
The dolphinlike features of this ichthyosaur are evident from its fossilized remains. The powerful tail was half-moon-shaped, but here only the down-turned backbone is preserved.
dorsal vertebrae with attachment points for long ribs rib
long neck comprising 30 vertebrae
PLESIOSAUR
Cryptoclidus eurymerus is a mid-Jurassic plesiosaur. It has a small head, long neck, and short tail, which is typical of shallowwater forms. Its sharp teeth indicate that it ate small fish or shrimplike crustaceans. bones of shoulder girdle are flattened into thin plates
pointed, interlocking teeth trap prey
paddlelike hind flipper large bones of pelvic girdle
TIMELINE OF EARTH HISTORY 4,000 MYA
Million 4,100 MYA years ago CRYPTOZOIC EON 4,500–542 MYA
first organic molecules
3,500 MYA
3,000 MYA
first stromatolites
50–14 MYA Return to Water
OCEAN LIFE
ANCIENT WHALE SKELETON
This skeleton has been exposed in a desert in Saccao, Peru. Whales evolved over the last 50 million years, so this area must have been an ancient sea at some point in this period. SKULL WITH A BLOWHOLE
The nostrils of Prosqualodon davidi are positioned on top of the head, forming a blowhole. This feature proves that this fossil skull is from a primitive whale.
Following the mass extinction that saw the demise of marine reptiles, some mammals that had evolved on land began returning to the water. Around 50 million years ago, the oceans started to resemble modern oceans in terms of their geographical positions and fauna. The ancestors of whales, however, were unlike their modern counterparts. The earliest whale, Pakicetus, was probably a close relative of the hoofed mammals (ungulates), but it is known only from its skull. Ambulocetus, which means “walking whale,” is another early form. It had few adaptations for living in water and probably still spent much time on land. The productivity of the oceans increased, whales diversified, and other marine mammals appeared. Whales similar to today’s toothed whales appeared first, and a few million years later, baleen whales evolved. By 24 million years ago, baleen whales had reached today’s giant sizes, suggesting that plankton was present in vast numbers for them to feed on. Only 14 million years ago, pinnipeds and sirenians (dugongs and manatees) evolved. It is thought that pinnipeds arose from a family of carnivores not unlike otters. Their present-day forms are seals, sea lions, and walruses.
2,500 MYA first microfossils
2,200 MYA
first eukaryotes and multicellular algae
HUMAN IMPACT
LIFE ON THE MOVE Humans have long had a profound effect on the oceans through pollution, overfishing, and linking oceans with canals. People have also transported marine organisms all over the world, in and on their ships, without knowing what long-term impact this will have.
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Today: Life in Modern Oceans
GRAY REEF SHARK
Like its close relative the Caribbean reef shark (below), this shark lives in warm, shallow waters, near coral atolls and in adjacent lagoons. It is found in the Indian and Pacific oceans, but it is cut off from the Atlantic.
CARIBBEAN REEF SHARK
PA L 65 AE –2 OG 3.3 E M NE YA NE O 23 GE .3– N QU 1.8 E 1.8 ATE MYA –P RN RE AR SE Y NT
CR 14 ETA 2– CE 65 O M US YA
JU 19 RAS 9.5 S –1 IC 42 M YA
Like the gray reef shark (above), this species lives in shallow water near coral reefs. Its range is isolated from the Indo-Pacific by the deep, cold ocean around South Africa, so it is restricted to warm parts of the Atlantic, from the Caribbean to Uruguay.
M I 35 SSI 4– SS 32 IP 3 M PI YA AN PE NN 32 S 3– Y 29 LV 0 M AN YA IAN PE R 29 MI 0– AN 25 2M YA TR I 25 ASS 2– IC 19 9.5 M YA
SI L 44 UR 3– IA 41 N 8M YA DE 41 VON 8– IA 35 N 4M YA
OR 49 DOV 0– IC 44 IA 3M N YA
CA 54 MB 2– RI 49 AN 0M YA
ED 63 IAC 5– AR 54 AN 2M YA
We know much more about life in today’s oceans because, as well having entire organisms to study, we can also observe life cycles, locomotion, and behavior. Each of the five oceans supports a wide variety of life. Some species are very specialized and are restricted to a small area, while others are migratory or generalists and have a wider distribution. Sometimes, closely related species live in the same habitat in different oceans, separated by land or other physical barriers (see right). By studying living organisms and the characteristics of the water they live in, scientists can also better understand ancient ocean environments and organisms. The deep ocean is still poorly known, but it contains an ecosystem that could be crucial to our understanding of life—black and white smokers (see p.188). Isolated from sunlight and from the surrounding water by a steep thermal gradient, it is possible that this is the type of environment in which life first evolved 3.5 billion years ago.
earliest sharks appear fourth mass extinction Cambrian Explosion: rapid evolution of body forms
635
600
550
first lobe-finned fish appear
first armored fish appear
450
500
400
earliest penguins
third mass extinction
350
300
first turtles appear
250
200
150
100
second mass extinction plactodonts (earliest marine reptiles) appear
earliest jawless fish, representing the first vertebrates, appear
1,500 MYA
2,000 MYA
plesiosaurs replace placodonts ichthyosaurs appear
1,000 MYA
700 MYA
mosasaurs replace ichthyosaurs
635 MYA PHANEROZOIC EON 542 MYA–PRESENT
first fossil evidence of mineralized skeletons
Mass Extinctions
This ammonite species is one of the few to survive the lateTriassic mass extinction event.
fifth mass extinction kills last ammonites
PRESENT DAY
beginning of the Ediacaran period, which soon features the first multicellular life
VOLCANIC ARMAGEDDON
Volcanic activity in the western Ghats of India is now thought to have been a factor in the most recent mass extinction. The eruptions would have caused destruction and climate change on a global scale.
OCEAN LIFE
The history of life is punctuated by five mass extinctions—catastrophic events in which many life forms died out. The first occurred 443 million years ago, when prominent marine invertebrates disappeared from the fossil record. About 368 million years ago, global cooling and an oxygen shortage in shallow seas caused about 21 percent of marine families to disappear, including corals, brachiopods, bivalves, fishes, and ancient sponges. At the end of the Permian Period, 252 million years ago, the cooling and shrinking of oceans killed over half of all marine life. Another mass-extinction event at the end of the Triassic Period, 199.5 million years ago, caused major losses of cephalopods, especially the ammonites. The fifth extinction, 65 million years ago, caused the demise of the dinosaurs; in the oceans, it caused the giant marine reptiles to disappear. The next mass extinction is likely to be a result of human activity. AMMONITE FOSSIL
50 whales diversify
first mass extinction
Ediacaran fossils show early multicellular life
whales evolve from terrestrial mammals
LIFE ON EARTH was once thought to fall into
five great kingdoms—the animals, plants, and fungi, and the two microscopic kingdoms, the protists and the bacteria. Scientists are now looking at life ever more closely, and each discovery expands our perspective on life’s vast variety. Many experts now consider that the familiar life-forms, plants and animals, represent just two of 30 or more kingdoms. The oceans are the ancestral home of life and are still home to all major groups of animals. Although plants are far more diverse on land, their place is taken in the oceans by a range of seaweeds and microorganisms. The following section showcases the entire range of ocean life. It is organized into kingdoms and further divided into the smaller units used by scientists to order and understand nature.
KI N GD OMS OF O C E A N L IF E SUCCESS IN WATER
This violet-spotted reef lobster is a flamboyant example of one of the marine success stories of the animal kingdom—the varied and abundant crustaceans. The crustaceans as a group include crabs, crayfish, shrimp, and some of the most common members of the zooplankton.
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BACTERIA AND ARCHAEA
Bacteria and Archaea THE SMALLEST ORGANISMS ON EARTH
are the bacteria and their relatives, the archaea. Bacteria occupy virtually Archaea all oceanic habitats, whereas many archaea are confined KINGDOMS 13 to extreme environments, such as deep-sea vents. Bacteria SPECIES Many millions and archaea play vital roles in the recycling of matter. Many are decomposers of dead organisms on the ocean floor. Others are remarkable in being able to obtain their energy from minerals in the complete absence of light.
OCEAN LIFE
DOMAINS Bacteria
Anatomy
Habitats
Bacteria and archaea are singlecelled organisms that are far smaller than any other, even protists. Most have a cell wall, which, in bacteria, is made from a substance called peptidoglycan. None has a nucleus or any of the other cell structures of more complex organisms (eukaryotes). Some bacteria and archaea can move by rotating threads called flagella; others have no means of propulsion. Scientists separated the Archaea and Bacteria groups on the basis THRIVING IN THE RIGHT CONDITIONS of chemical differences in their cell The bacterium Nitrosomonas forms colonies make-up. All living cells contain wherever there is enough ammonia and oxygen in the water. tiny granules (ribosomes), which help to make proteins, but those in archaea are differently shaped to those in bacteria. The oily substances that make up their cell membranes are also different. Additionally, archaea have special molecules associated with their DNA that protect them in the harsh environments in which they live. Scientists now think that their chemical HEAT-LOVING ARCHAEA differences are sufficiently important to rank Most archaea can adapt Archaea as a distinct evolutionary branch of to extreme conditions. This life. Initially considered to be primitive, the heat-loving example, GRI, archaea are now thought to be closer to the was ejected from the sea ancestors of eukaryotes than are the bacteria. floor in an undersea eruption.
Bacteria are found throughout the ocean environment, because nearly all habitats provide them with the materials necessary to obtain energy. Most bacteria get energy by breaking down organic matter. Much of this matter accumulates on the ocean floor and provides excellent conditions for the decomposer bacteria. Bacteria are also found in large numbers in the water column, feeding on suspended matter. A few kinds of bacteria, such as cyanobacteria, can photosynthesize and so live nearer to the surface, in brightly lit waters. Some form colonies and build huge structures, called stromatolites, near the shore. Many archaea and bacteria can live in extreme conditions, such as high temperatures, water with high acidity, or low oxygen levels. For example, archaea and bacteria live around deep-sea vents, getting their energy from chemical reactions of methane and sulphide compounds ejected by the vents. Others survive in the very high concentrations of salt on some sea shores.
HYPER-SALINE CONDITIONS
The hyper-saline water of Hamelin Pool, west Australia, is one of only three places where stromatolites are found. The rocks are formed by the cyanobacteria cementing sediment particles together.
PEOPLE
CARL WOESE Born in New York Carl Woese (1928—2012) was the microbiologist who is responsible for the current division of living organisms into three domains, Archaea, Bacteria, and Eucarya, on the basis of his research into the RNA (a chemical related to DNA, called ribonucleic acid) found in ribosomes. Woese put forward his new classification in 1976, but it was not until the 1980s that his hypothesis was accepted.
LIVING ON THE SEA BED
Bacterial mats form on the sea bed where oxygen supply is low. This mat of Beggiatoa sp. is at the mouth of the Mississippi, USA.
BACTERIA AND ARCHAEA DOMAIN BACTERIA
DOMAIN BACTERIA
Oscillatoria willei SIZE
DISTRIBUTION
DOMAIN BACTERIA
Trichodesmium erythraeum 1–10mm per colony
DISTRIBUTION
SIZE
Tropical waters
Once known as blue-green algae, cyanobacteria are bacteria that are able to use photosynthesis to make foods in a similar way to plants. Oscillatoria willei and other related cyanobacteria occur in rows of similarly sized cells that form filaments called trichomes. Many trichomes are enveloped in a firm casing, but in Oscillatoria the casing is thin or may be absent altogether, which allows the filaments to glide quickly forwards, backwards, or even to rotate. Some species of Oscillatoria can fix nitrogen but, unlike Trichodesmium (below), they may not have cells specialized for the purpose.
SIZE
Calothrix crustacea
Filament length 0.13mm
Tropical and subtropical seas
worldwide
Filament length 0.15mm
DISTRIBUTION
Fragments of filaments, called hormogonia, which consist of dozens of cells, sometimes break off and glide away to establish new colonies. These bacteria may cause skin irritations in humans who come into contact with them in tropical waters.
Worldwide
Forming single filaments or small bundles, bacteria of the genus Calothrix are widespread in oceans everywhere. Unlike those of Oscillatoria and Trichodesmium (left), the filaments of Calothrix crustacea have a broad base and a pointed tip that ends in a transparent hair. The filament has a firm or jelly-like coating, which is often made up of concentric layers that may be colourless or yellow-
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brown. Unusually, the filament grows in much the same way as a plant root, its growth being confined to a special region just behind the tip, called a meristem. Sometimes, the filament sheds the tapering tip above the growth region, enabling Calothrix to reproduce asexually by casting off fragments called hormogonia from the meristem. These fragments are able to form new filaments far away from the parent. These kinds of cyanobacteria often form slimy coatings on coastal rocks and seaweeds. At least one species of Calothrix is known to make up the photosynthetic part of some rocky shore lichens, such as Lichina pygmaea (p.255).
oceanic food chains. The bacteria form long colonial filaments, in which some cells carry out nitrogen fixation, while others are specialized for photosynthesis. These tasks must be separated because the oxygen by-product that results from photosynthesis would interfere with the nitrogen-fixing process, so they cannot both occur in the same cell.
Individual filamentous colonies of the cyanobacteria Trichodesmium erythaeum are just visible to the naked eye, and these bacteria have traditionally been known as sea sawdust by mariners. Under warm conditions, the bacteria is able to multiply extremely rapidly to create massive blooms that may have such an extent that they are visible from space. This is a prolific nitrogen-fixing bacterium that harnesses about half of the nitrogen passing through
DOMAIN BACTERIA
Vibrio fischeri SIZE
0.003mm cell length
DISTRIBUTION
Worldwide
Many marine organisms, particularly those in the deep sea, make use of bioluminescence, the biochemical emission of light. Many of these creatures depend on bacteria, such as the rod-shaped Vibrio fischeri, to generate the light, and in these cases
the bacteria live within the body of their host in a mutually beneficial relationship. The bacteria produce light using a chemical reaction that takes place inside their cells. Vibrio fischeri also occurs as a free-living organism, moving through water by means of a flagellum and feeding on dead organic matter. The distinctive, comma-shaped cells seen in Vibrio fischeri, below, are characteristic of the genus. Other Vibrio species (which are not luminescent) are responsible for the potentially fatal disease cholera.
EYE LIGHTS Eyelightfish (such as the one shown below) have light-emitting organs called photophores under each eye. The light is produced by colonies of Vibrio fischeri living in the photophores. The light organs can be covered and uncovered, and may be used as an aid to recognition and communication between fish of this species. The ability to emit light may also play a part in prey capture and the avoidance of predators.
DOMAIN ARCHAEA
Halobacterium salinarium SIZE
0.001–0.006mm
DISTRIBUTION
Dead Sea and other hypersaline areas
of the world
OCEAN LIFE
Archaea that have adapted to live in waters with exceptionally high salt concentrations are called halophiles. One example of this type of organism is Halobacterium salinarium, which is rod-shaped, produces pink pigments called carotenoids, and forms extensive areas of pink scum on salt flats. The cell membranes of halophiles contain substances that make them more stable than other types of cell membrane, preventing them from falling apart in the high salt concentrations in which they live. Their cell walls are also modified, for the same function. These archaea obtain nourishment from organic matter in the water. In addition, their pigments absorb some light energy, which the bacteria then use for fuelling processes within the cells.
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CHROMISTS
Chromists
GIANT CHROMISTS
The largest chromists are brown seaweeds called kelps. Firmly attached to rocks, they are seen flourishing in this clear, shallow water along the coast of South America.
MOST CHROMISTS
are microscopic in size, including many of the PHYLA At least 11 important photosynthetic SPECIES 31,200 plankton groups such as diatoms. However, brown seaweeds, which can grow to a huge size, are also chromists and unlike green seaweeds are not considered to be true plants. Like true plants, chromists make their own food by photosynthesis but they use different pigments to capture the sun’s energy. DOMAIN Eucarya
Anatomy Brown seaweeds and the various kinds of microscopic chromists all have very different shapes and structures. Their shared characteristics are at the cellular level. Chromists use an additional type of chlorophyll (a pigment) for photosynthesis and, unlike true plants, the food they manufacture is not stored as starch. They also have other colored pigments, which give brown seaweeds their color. Like green and red seaweeds, brown seaweeds do not need roots to absorb water and nutrients. Some have stiff stalks (stipes) and gas-filled bladders (pneumatophores) that hold up their leaflike fronds to the light. Some microscopic chromists, such as diatoms, have a rigid outer skeleton and float passively. Others, including dinoflagellates, can propel themselves along using whiplike threads called flagella.
BROWN SEAWEED
In place of roots, brown seaweeds have a holdfast which acts as an anchor. In this one, called Laminaria hyperborea, the holdfast is a small disk. DIFFERENT SHAPES
There are thousands of species of diatoms. Each has a differently shaped silica skeleton.
OCEAN LIFE
CHROMIST OR PROTIST?
DRIFTING PLANKTON
Radiolarians use their delicate arms to trap food particles and aid buoyancy as they float in the plankton.
Scientists used to place all single-celled eukaryotes including microalgae, protozoans, and chromists in one taxonomic group, the Protista. Today the term protist is used informally to indicate any of these diverse unicellular organisms. But there is no final agreement as to whether all the organisms included as chromists in this book really belong together in this kingdom.
CHROMISTS
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Habitats Chromists live in every ocean in the world. Others need sunlight to photosynthesize, so the microscopic forms, most of which are phytoplankton, are only found in the well-lit surface layers. Some, such as foraminiferans can live on the seabed, along with brown seaweeds, which often dominate rocky seashores in cooler climates. Great underwater forests of brown kelps grow in colder waters but they and most other brown seaweeds do not usually grow below about 66 ft (20 m). Some brown seaweeds grow unattached in sheltered lagoons and sea lochs, and a few grow in salt marshes anchored in mud.The brown sargassum or gulfweed, Sargassum natans, is unusual in that it floats at the surface of the open ocean, forming the basis for a unique ecosystem (see p.238). PLANKTON BLOOM
The milky bloom in this satellite image off Cornwall, UK, is of the coccolithophore Emiliania huxleyi, which has multiplied rapidly in favorable conditions.
Life Strategies Most single-celled chromists drift in the ocean surface layers as phytoplankton. They reproduce by splitting into two and some can do so very rapidly (see above). They can also reproduce sexually, forming the equivalent of egg and sperm cells. Their number increases dramatically with the warmth and longer days of spring and summer. Many seashore brown seaweeds produce slippery mucus, both to protect from drying out and to deter grazers. While some brown seaweeds are annuals, living for only a year, large species are usually perennial. NEW KELP GROWS FROM OLD
A new, yellow frond is growing from the top of this kelp stipe. The old frond, which will drop off, is covered in white animals called bryozoans, which block vital photosynthesis. upper valve of pillbox-shaped diatom
DIATOM DIVISION
When this diatom Coscinodiscus granii divides, the two halves will separate such that each daughter cell inherits one valve from its parent and creates another itself.
HUMAN IMPACT
FARMING THE OCEANS Seaweeds are harvested wild, but more are now grown in ocean nurseries and farms, especially in Asia. They are used for food, and seaweed extracts are used in a wide range of products—for example, in gels as a stabilizer, in cosmetics and pharmaceuticals, in beer-making, and as a fertilizer. SEAWEED HARVEST IN ZANZIBAR
Many seaweeds grow readily on floating rafts, as seen here. They flourish in strong light, away from grazing invertebrates, and provide vital income for coastal communities.
OCEAN LIFE
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CHROMISTS INFRAPHYLUM DINOFLAGELLATA
Noctiluca scintillans DIAMETER HABITAT
Up to 1/16 in (2 mm)
Suface waters
DISTRIBUTION
Worldwide
Also known as sea sparkle, Noctiluca scintillans is a large, bioluminescent dinoflagellate that floats near the surface of the ocean, buoyed up by its oily cell contents. It is one of the naked dinoflagellates, which do not have a protective outer theca (shell). Like all dinoflagellates, it has two flagella but one is tiny. This species feeds on other plankton, and its second large flagellum helps sweep food particles toward it, which it then engulfs. Other dinoflagellates also feed in this way but there are many that are photosynthetic.
BIOLUMINESCENCE Floating just below the surface of the water at night, dinoflagellates, and in particular Noctiluca scintillans, are the most common cause of bioluminescence in the open ocean. Millions of Noctiluca scintillans cells twinkle in the waves, hence the common name sea sparkle. The blue-green light is emitted from small organelles within the cells and is generated by a chemical reaction. Unlike many bioluminescent fish, it does not depend on light-emitting bacteria.
INFRAPHYLUM DINOFLAGELLATA
Gymnodinium pulchelum DIAMETER HABITAT
0.025 mm
Surface waters
Temperate and tropical waters above continental shelves, and Mediterranean DISTRIBUTION
Some red-tide organisms such as Gymnodinium pulchellum produce toxins that affect the nervous system and the clotting properties of the blood, causing high mortality among fish as well as
invertebrates.The cause of red tides is not well understood but some scientists think they may be influenced by coastal pollution providing nutrients that might otherwise be in short supply and so normally limit the population size. Rapid reproduction by simple cell division results in huge numbers of Gymnodinium pulchellum being present in the water, turning it a characteristic brown-red color, as shown here in the seas around Hong Kong. Unlike many other types of dinoflagellate, this species lacks a test and also produces food by photosynthesis.
INFRAPHYLUM DINOFLAGELLATA
Neoceratium tripos
Strombidium sulcatum
LENGTH
0.2–0.35 mm
DIAMETER
HABITAT
Surface waters
HABITAT
DISTRIBUTION
Worldwide
0.045 mm
Surface waters
DISTRIBUTION
apical horn
lateral horns aid floatation
OCEAN LIFE
PHYLUM CILIOPHORA
The unique three-pronged shape of the dinoflagellate Ceratium tripos makes it easy to identify among the phytoplankton, where it is one of the dominant organisms. Although this species is usually solitary, several individuals may be seen together, attached to each other by the single apical horn. This occurs when a cell divides and the daughter cells remain linked in short chains. Ceratium tripos is sometimes parasitized by other protists.
Atlantic, Pacific, and Indian Oceans
Organisms such as Strombidium sulcatum are classified as ciliates because the cell membrane has many hairlike projections, called cilia, that are used in locomotion. In Strombidium sulcatum, the cilia are restricted to a collar at one end of its spherical body, which has no shell. Ciliates are the most complex of all protists and have two nuclei in their single cell, a macronucleus and a micronucleus. For most of the time Strombidium sulcatum reproduces asexually by splitting both nuclei and the cell into two. Periodically it must undergo a type of sexual reproduction called conjugation. Two individuals partially merge so that once the micronuclei have divided they can each obtain one part from the other. They then separate and each forms a new macronucleus from its micronucleus and then divides. Ciliates and dinoflagellates share some cell characteristics, and both belong to a group known as the alveolates.
CHROMISTS PHYLUM RADIOZOA
Cladococcus viminalis DIAMETER HABITAT
0.08mm
Surface waters
DISTRIBUTION
Mediterranean
Radiolarians produce extremely complex silica tests of spines and pores that are laid down in a well-defined
PHYLUM HAPTOPHYTA
Emiliania huxleyi DIAMETER HABITAT
0.006 mm
Surface waters
DISTRIBUTION
Atlantic, Pacific, and Indian Oceans
Emiliana huxleyi is a protist belonging to a group of haptophytes commonly known as coccolithophores. The name comes from a covering of intricately sculptured calcite plates called coccoliths with patterns that are unique to each species. Like some other protists, E. huxleyi can multiply
geometric pattern. The spines aid buoyancy and the pores provide outlets for cell material, called pseudopodia, which engulf any food that becomes trapped on the spines and carry it to the centre of the cell to be digested. Cladococcus viminalis is a polycystine radiolarian, which are the most commonly fossilized radiolarians and are frequently found in chalk and limestone rocks. very quickly in favorable conditions and form blooms that can cover areas of up to 38,600 square miles (100,000 square km). These blooms are visible from space because the coccoliths act like tiny mirrors and reflect sunlight so the water they are in appears a milky white. By reflecting light and heat and by “locking up” carbon in their calcite coccoliths, they help reduce ocean warming. They have been found worldwide in chalk deposits dating from 65 million years ago. The famous white cliffs of Dover in the UK are mainly formed from coccolith plates.
PHYLUM FORAMINIFERA
Hastigerina pelagica LENGTH 1/4 in HABITAT
(6 mm)
Warm waters at depth of 660 ft (200 m)
DISTRIBUTION Subtropical and tropical waters of North Atlantic and western Indian Ocean
Foraminiferans are unicellular organisms found only in marine habitats. Hastigerina pelagica is one of the larger forms. It is often pinkish red and has a calcareous test with several globular-shaped chambers from which radiate calcite spines
Ethmodiscus rex DIAMETER 1/16–1/8 in (2–3 mm) HABITAT
Warm, nutrient-poor water
DISTRIBUTION
Open ocean worldwide
Ethmodiscus rex is the largest of all diatoms and can be seen with the naked eye. It is a single cell with a rigid cell wall, called a test, which is impregnated with silica and covered in regular rows of pits. The test is made up of two disk-shaped halves, called valves, which fit tightly together. Because each diatom has a unique test, Ethmodiscus rex can be easily identified in the fossil record. It is found in rocks that date from the Pliocene and the fossils can be up to 5 million years old. The cells need to remain near the water surface in order to utilize the Sun’s energy for food, which they do by transforming the
PHYLUM OCHROPHYTA
Chaetoceros danicus LENGTH HABITAT
0.005–0.02 mm Surface waters Worldwide
First described in 1844, Chaetoceros is one of the largest and most diverse genera of marine diatoms, containing nearly 200 species. Chaetoceros danicus is a colonial form, and groups of seven cells are not uncommon (as shown here). It is easily recognized
globular-shaped chamber of calcareous test
Dictyocha fibula LENGTH
0.045 mm
HABITAT
Surface waters
Atlantic, Mediterranean, Baltic Sea, and eastern Pacific off coast of Chile DISTRIBUTION
The golden-yellow pigments visible in this image of Dictyocha fibula are typical of two groups of golden algae known as Chrysophyceae and Dictyochophyceae (this species). The word Dictyocha means “net” and refers to the large windows in the silica test. Fewer than 20 species of Dictyocha are alive today. They
products of photosynthesis into oily substances that increase their buoyancy. Ethmodiscus rex can reproduce sexually but, if conditions are favorable, it multiplies rapidly, simply by dividing into two. Over a 10-day period, one individual that divides three times a day can theoretically have more than 1.5 billion descendants.
valve forms one half of test
rigid cell wall (test)
are all that remains of a group of organisms that flourished more than 5 million years ago. Their fossils are abundant in some Miocene deposits. golden yellow pigments used in photosynthesis
projection from silica test
OCEAN LIFE
DISTRIBUTION
because it has highly distinctive long, stiff hairs, called setae, which project perpendicularly from the margins of its test. and have prominent secondary spines along their length. Chloroplasts, which contain pigments used in photosynthesis, are numerous and found inside both the cell and the setae. The setae are easily broken and if large quantities lodge in the gills of a fish, they may kill it. The secondary spines anchor the setae to the sensitive gill tissue causing irritation, and the fish reacts by producing mucus. Eventually, it dies from suffocation.
covered with cytoplasmic strands (pseudopodia) for collecting food. Hasterigina pelagica is unique in surrounding its test with a gelatinous capsule of tiny frothy bubbles, which is thought to aid buoyancy. Dinoflagellates sometimes live on the surface of the capsule and up to 79 have been counted on a single individual, although 6–10 is more usual. The relationship between the two organisms is not clearly understood because Hasterigina pelagica is carnivorous, yet the dinoflagellates are unharmed.
calcite spines aid buoyancy
PHYLUM OCHROPHYTA
PHYLUM OCHROPHYTA
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CHROMISTS PHYLUM OCHROPHYTA
Limey Petticoat Padina gymnospora HEIGHT
Up to 10cm (4in)
Rock pools and shallow subtidal rocks
HABITAT
WATER TEMPERATURE
20–30˚C (68–86˚F) DISTRIBUTION Coasts in tropical and subtropical areas worldwide
Padina is the only genus of brown seaweeds to have calcified fronds, hence this species’ common name of Limey Petticoat. The reflective chalk shows as bright white concentric bands on the upper surface of the fan-shaped fronds. The fronds are only 4–9 cells thick and curled inwards. Older fronds may become split into wedge-shaped sections. This species is widespread in tropical seas, often growing in masses on shallow subtidal rocks, and on old coral and shells.
PHYLUM OCHROPHYTA
Giant Kelp Macrocystis pyrifera LENGTH
45m (150ft)
Rocky sea beds, occasionally sand
HABITAT
WATER TEMPERATURE
5–20˚C (41–68˚F) Temperate waters of southern hemisphere and northeastern Pacific
DISTRIBUTION
Giant Kelp (pictured on pp.240–41) is the largest seaweed on Earth. It can grow at the rate of 60cm (24in) per day in ideal conditions, and reaches lengths of over 30m (100ft) in a year. Giant
PHYLUM OCHROPHYTA
Oyster Thief Colpomenia peregrina DIAMETER
Up to 10cm
(4in) Intertidal and subtidal rocks and shells
HABITAT
WATER TEMPERATURE
6–28˚C (49–83˚F) Coasts of western North America, Japan, and Australasia; introduced in Atlantic
DISTRIBUTION
The Oyster Thief gets its unusual name from its habit of growing on shells, including commercially grown oysters. The frond is initially spherical and solid, but as it grows, it becomes irregularly lobed and hollow and fills with gas. Sometimes, this can make it sufficiently buoyant to lift the oyster,
PHYLUM OCHROPHYTA
PHYLUM OCHROPHYTA
Landlady’s Wig
Sea Palm
Desmarestia aculeata LENGTH
Postelsia palmaeformis Up to 1.8m (6ft)
LENGTH
Up to 60cm
(24in)
Subtidal rocks, and kelp forests
HABITAT
HABITAT
Wave-exposed
shores
WATER TEMPERATURE
0–18˚C (32–64˚F)
WATER TEMPERATURE
8–18˚C (46–64˚F) DISTRIBUTION
Near coasts in temperate, cold, and
DISTRIBUTION
West Coast of North America
polar regions
OCEAN LIFE
This large seaweed has narrow brown fronds with many side-branches. Its bushy appearance is the reason for its common name of Landlady’s Wig. The smallest branches are short and spinelike, hence the species name aculeata, which means “prickled”. In summer, the whole plant is covered with delicate branched hairs. This species is particularly abundant on boulders and in kelp forests disturbed by waves.
Sea Palms are kelps, which are large brown seaweeds that belong to the order Laminariales, as does Giant Kelp (above). Unusually for a kelp, Sea Palm grows on the midshore, where it forms dense stands on wave-exposed coasts. It has a branched holdfast, and a stout, hollow stalk, which stands erect when the tide is low. The top of the stalk is divided into many short, cylindrical branches, each of which bears a single frond up to 25cm (10in) long, with toothed margins and deep grooves running down both faces. Spores are released into the grooves and drip off the frond tips onto the holdfasts and nearby rocks at low tide, so that the developing seaweeds grow as dense clumps. Some Sea Palms attach to mussels and are later ripped off during storms, making more rock available for other Sea Palms to grow.
Kelp normally grows at a depth of 10–30m (30–100ft), but it can grow much deeper in very clear water. The huge branched holdfast, which is about 60cm (24in) high and wide after three years, is firmly attached to the sea bed. From it, a number of stalks (or stipes) stretch towards the surface, bearing many strap-like fronds, each buoyed by a gas-filled bladder. The stem and fronds continue to grow on reaching the surface, floating as a dense canopy. Giant Kelp has a two-phase life cycle. Fronds (sporophylls) at the base of the kelp produce spores that develop into tiny creeping filaments. The filaments produce eggs and sperm, which combine to produce embryonic kelp plants. which is not fixed to the sea bed, and they may both be carried away by the tide. This seaweed has a thin wall with only a few layers of cells. The outer layer is made of small, angular cells which contain the photosynthetic pigments that give the Oyster Thief its brown colour.
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CROFTER’S WIG In very sheltered bays and sea lochs, detached pieces of “normal” Knotted Wrack will continue to grow, lying loose on the sea bed. In situations where the fronds are alternately covered by salt and fresh water, they divide repeatedly to form a dense ball that has no bladders or reproductive structures. This unattached form, which is known as Crofter’s Wig, appears very different to the attached form, even though it is genetically identical.
PHYLUM OCHROPHYTA
Knotted Wrack Ascophyllum nodosum LENGTH
Up to 3m (10ft)
Sheltered seashores
HABITAT
WATER TEMPERATURE
0–18˚C (32–64˚F) DISTRIBUTION Coasts of northwestern Europe, eastern North America, and north Atlantic islands
PHYLUM OCHROPHYTA
Neptune’s Necklace Hormosira banksii LENGTH
Up to 30cm
(12in) Lower shore and subtidal rocks
HABITAT
WATER TEMPERATURE
10–20˚C (50–68˚F) Coasts of southern and eastern Australia and New Zealand DISTRIBUTION
Knotted Wrack belongs to a group of tough brown seaweeds that often dominate rocky seashores in cooler climates. It is firmly attached to the rocks by a disc-shaped holdfast, from which arise several narrow fronds that often grow to 1m (3ft) in length, and exceptionally to 3m (9ft) in very sheltered situations. Single oval bladders grow at intervals down the frond. The fronds produces about one bladder a year, so the seaweed’s age can be roughly estimated by counting
PHYLUM OCHROPHYTA
Japweed Sargassum muticum LENGTH
2–10m
(6–33ft) Intertidal and subtidal rocks and stones
HABITAT
WATER TEMPERATURE
5–26˚C (41–79˚F) Coasts of Japan, introduced in western Europe and western North America DISTRIBUTION
Japweed can reproduce all year round and forms dense stands in quiet waters. Native to Japan (hence its common name), it was accidentally introduced to western North America and Europe, and is steadily expanding its range in these areas. It outcompetes other seaweeds and in these regions is regarded as an invasive species. This long, bushy seaweed has numerous side-branches, which have many leaflike fronds up to 10cm (4in) long. The fronds bear small, gas-filled bladders, either singly or in clusters.
OCEAN LIFE
Neptune’s Necklace is one of the many brown seaweeds endemic (unique) to New Zealand and the cooler waters around Australia. Its distinctive fronds, which look like a string of brown beads, are made up of chains of ovoid, hollow segments joined by thin constrictions in the stalk. Small reproductive structures are scattered over each “bead”. Dense mats composed almost entirely of this one species can be found on seashore rocks. The fronds are attached to the rock by a thin, disc-shaped holdfast. Neptune’s Necklace also lives unattached among mangrove roots. The shape of its segments varies according to habitat. They are spherical and about 2cm (3/4 in) wide in fronds growing on sheltered rocks, mussel beds on tidal flats, or in mangrove swamps. Fronds growing on subtidal rocks on moderately exposed coasts have smaller segments that are just 6mm (1/4 in) long.
a series of bladders. The bladders hold the fronds up in the water so that they gain maximum light, which is an advantage in the often turbid waters where Knotted Wrack grows. This also makes it harder for grazing snails to reach the fronds when the tide is in. The dark brown fronds may be bleached almost to yellow in summer. Reproductive structures that look like swollen sultanas are borne on short side-branches, and orange eggs can sometimes be seen oozing from them.
GIANT KELP
This enormous seaweed can grow at a rate of 20 in (50 cm) per day in favorable conditions, such as the relatively cold water off California (shown here). Air bladders help keep the kelp’s blades afloat as they grow upward toward the surface, where there is an enhanced supply of light and nutrients.
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PLANT LIFE
Plant Life
HUMAN IMPACT
BEACH PLANTS
PLANTS FORM A GREAT
kingdom of life-forms, all of which use the pigment chlorophyll to KINGDOM Plantae fix carbon dioxide from the atmosphere into SPECIES 315,000 organic molecules, using energy from sunlight. Most organisms in the plant kingdom are “higher” plants, which evolved on land and remain land-based. Of these, several unrelated families of flowering plants (see p.250) have since returned to the sea or taken up residence on the coast. The plant kingdom, as defined in this book, also encompasses more primitive organisms that first evolved in water—the microscopic green algae (microalgae) and the green and red seaweeds (macroalgae). Brown seaweeds appear very similar but may not be closely related to true plants and are classified in this book in the phylum Chromista (see p.236). DOMAIN Eucarya
Marine Plant Diversity
TEMPERATE MARINE PLANTS
dune flower
mangrove tree
The sand crocus of the Canary Islands grows only in a few coastal locations on Lanzarote and Fuerteventura. It is protected, but threatened by tourism development.
Seagrasses are abundant in tropical lagoons, and green seaweeds include calcified species. Mangroves line estuaries and creeks, and other flowering plants, including shrubs and trees, colonize the back of sandy beaches. Although not shown here, seaweeds may grow seasonally on rocky shores.
H AC
BE
P
M WA ES
coconut palm
V RO
NG
MA
high tide mark intertidal zone of beach mermaid’s wine glass alga, Acetabularia
sea campion
S
NE
DU
SAND CROCUS
TROPICAL MARINE PLANTS
Plants are united by their use of chlorophyll for photosynthesis. The higher plants include several major land-based groups, including ferns and conifers. Because higher plants evolved on land, they are adapted to life in air and to fresh water. They have tissues bearing vessels that transport water and food. Of the higher plants, it is mainly the flowering plants (the largest group) that have invaded marine habitats. Along with mosses, they inhabit the coastal fringes, with only the seagrasses being fully marine. Green seaweeds and microscopic green algae (microalgae) lack stems and roots, have neither woody tissues nor transport vessels, and are mostly aquatic. Temperate seas are rich in phytoplankton, including green algae. Green algae commonly grow on rocks and red algae on mud flats. F LIF Seagrasses have true roots AC E and live in sediment in the S shallow subtidal and intertidal zones, and in brackish lagoons. Above high water, cliffs, sand dunes, and salt marshes are home to flowering plants, mosses and lichens.
Beach plants grow in places used by humans for recreation. We can coexist, especially when people use paths in coastal dune areas. In fact, paths maintain low-growing plants, such as mosses, which might otherwise become overgrown. However, fragile dunes are damaged by erosion, and plants that grow only in a limited strip of coastal habitat are highly vulnerable to human development.
Halophila sea grass clear water with few algae
moss
SH
marram grass
OR
E
red seaweeds on intertidal rocks
Caulerpa seaweed green seaweeds on intertidal rocks
thrift
SH AR
TM AL
S
LA GO
ON
sea lettuce seaweed on boulders and bedrock
CO
RA
LR
EE
F
red seaweeds in subtidal zone Codium seaweed on boulders and bedrock
OCEAN LIFE
brackish lagoon
SEA ROCKET
Ulva seaweed in channel sea rocket above high tide intertidal zone
ED
ER ELT H -S E MI OR SE SH
eelgrass in sediment in subtidal zone surface water thick with green microalgae
ED ER ELT RE H S HO S
D SE PO RE X E O SH
A European member of the brassica family, sea rocket can grow on pure sand, just above high tide, where it traps sand, forming small foredunes. Its waxy leaves repel sea spray, while its stubby fruit pods are dispersed by the tide.
PLANT LIFE
243
Above High Water
PLANTS OF SHIFTING SHORES
Sea mayweed, with daisylike white flowers, and oyster plants, with their dark blue-green leaves, are salt-tolerant flowering plants that grow on semi-sheltered shores of shingle.
Above the reach of the highest tides, the environment is essentially terrestrial, but its proximity to the sea makes life hard for all but a few specialized flowering plants and lichens. In places, the coast is covered with dunes of sand blown from the seashore. Dunes are often alkaline, being rich in calcium carbonate from marine shells and maerl (see p.245). With few nutrients, dunes only support hardy colonizers tolerant of infertile and salty soil. Acidic dunes made from sand with little shell support many lichens and mosses such as golden dune moss. Marram, a grass with a fast-growing root and rhizome network stabilizes dunes and takes the first steps toward soil formation. Plants that can fix nitrogen in root nodules, such as the casuarina tree, have an advantage here. On rocky cliffs plants are safe from grazing animals but must withstand salty spray, drying winds and scanty soil. Plants such as sea campion have very long roots that reach deep into rock crevices for water draining from the land. Plants with succulent leaves and waxy cuticles can store the water they find.
ABOVE AND BELOW THE TIDE LINE
Marine plants range from coastal trees growing above the high tide mark, such as the Coconut Palm, to seagrasses, the only wholly aquatic marine flowering plants, below it.
Between the Tides Plants between the tides have to live both in and out of water in variable conditions sometimes hot, dry, and salty, and at other times drenched with cold, fresh rainwater. Intertidal zones support green and red seaweeds, seagrasses, and mangroves. Many green seaweeds are tolerant of brackish water and grow down the upper shore in strips following freshwater seeps and streams. Lower down, delicate red seaweeds thrive in pools or in the damp beneath tough brown seaweeds. Many are ephemeral, growing quickly in fair conditions and dispersing many spores. They may quickly cover tropical coasts where monsoons bring humid conditions, but then dry up and blow away when the sun returns. Seagrasses grow on lower intertidal flats; here the sediments retain moisture until the tide returns. In the tropics, mangroves colonize intertidal sediments, but only their roots are regularly submerged, while the rest of the plant remains in the air. In colder climates, salt-marsh vegetation develops on mudflats.
SUBMERGED SALT MARSH
The salt-tolerant sea pink grows in salt marshes, which develop on sheltered coasts in temperate regions. Like other salt-marsh plants, the sea pink is submerged only in the highest tides.
Aquatic Plants
OCEAN LIFE
Only microalgae, seaweeds, and seagrasses live permanently submerged in seawater. All seaweeds can absorb nutrients and gases over their whole body surface, so they do not need the transport systems of land plants, and their holdfast simply attaches them to the seabed. Seagrasses have a land-plant anatomy, so they need extra structures such as air spaces (lacunae) to aid gas exchange. Plants living in seawater can only thrive in the top few yards, because enough light for SEAWEED BED photosynthesis cannot Green seaweeds in the English Channel include penetrate beyond this. Codium (in the foreground) Many marine plants are and sea lettuce (lower right). tough to deter grazing They are growing here with animals or produce toxic an unrelated brown seaweed called serrated wrack. or distasteful chemicals.
244
RED SEAWEEDS
Red Seaweeds RED SEAWEEDS
are attractive marine plants and are found KINGDOM Plantae in shallow seas around DIVISION Rhodophyta the world. Their red color CLASSES 2 or more comes from the extra SPECIES 6,394 pigments they have in addition to green chlorophyll. While they appear to be plants, they differ in some cellular details and metabolism. Many scientists do not consider them to be true plants, but there is as yet no full consensus. DOMAIN Eucarya
Anatomy
Habitats and Distribution On the shore, red seaweeds mostly live at the lowest levels where they are less likely to dry out. In deeper water their additional pigments allow them to flourish in the dim blue light that remains and they extend deeper than brown and green seaweeds. Red (and brown) seaweeds are less abundant in tropical waters, an exception being the red oralline encrusting seaweeds, which play a very important role in cementing coral reefs.
BARBED COLONIZER
This red seaweed has barbed branches enabling detached fragments to hook onto other organisms and even ships’ hulls. It disperses in this way and has been transported outside its native range.
DULSE
The shape and form of red seaweeds is highly diverse but in general they are relatively small and delicate. Like green and brown seaweeds most have a holdfast, stipes (stems) and fronds that absorb water, nutrients, and sunlight. Coralline seaweeds have a heavily calcified frond, which is too hard for most grazers to eat. They look more like pink crusts or small corals than plants. Maerl forms unattached nodules that resemble twiggy coral lying on the seabed. Some red seaweeds have two phases, growing as a tiny long-lived crust and as an annual bushy frond. These look so different they were first described as separate species.
Red seaweeds rely on moving water to bring them nutrients and oxygen. The fingerlike fronds of this dulse increase its surface area while preventing it from tearing in rough conditions.
midrib
frond
stipe
holdfast
PERENNIAL SPECIES
This beautiful red seaweed is called sea beech. It grows new fronds each year from a perennial stipe, and reproduces from spores in winter.
DIVISION RHODOPHYTA
Small Jelly Weed Gelidium foliaceum LENGTH
2 in (5 cm) HABITAT
OCEAN LIFE
Intertidal rocks WATER TEMPERATURE
50–68˚F (10–20˚C) Coasts of southern Africa and southern Japan
DISTRIBUTION
There are many species of Gelidium worldwide, and they are difficult to identify because the plants can look very different depending on their habitat and whether they have been
grazed by seashore animals such as limpets. Ongoing work on molecular sequencing is gradually resolving some of these problems, and Gelidium foliaceum is one species that has recently been reclassified. It has a flattened, much lobed and curled frond, which grows in dense clumps on rocky seashores.The fronds are tough and cartilaginous, and the seaweed is attached to the rock at frequent intervals by small hairlike structures, or rhizoids, from a creeping stem, or stolon. This creeping habit is probably the main method of spreading, but some species of Gelidium also reproduce sexually. Some Gelidium species are a source of the gelatinous substance agar, which is used in cooking and microbiology.
RED SEAWEEDS DIVISION RHODOPHYTA
Laver Porphyra dioica LENGTH
Up to 20 in (50 cm) HABITAT
Intertidal rocks WATER TEMPERATURE
43–64˚F (6–18˚C) DISTRIBUTION Coasts of northeastern and western Europe and Mediterranean around Italy
DIVISION RHODOPHYTA
Irish Moss Mastocarpus stellatus LENGTH
7 in (17 cm)
Lower shore and subtidal rocks
HABITAT
WATER TEMPERATURE
32–77˚F (0–25˚C) Coasts of northeastern North America, northwestern Europe, and Mediterranean DISTRIBUTION
This tough red seaweed is common on exposed shores, often forming a dense turf on the lower shore. Its frond is attached to rock by a disk-shaped holdfast, from which arises a narrow stipe (stalk) that gradually expands
This species of red seaweed has only recently been separated from the very similar P. purpurea on the basis of how they reproduce. P. dioica is dioecious (male and female reproductive cells are on separate fronds), while P. purpurea is monoecious (male and female reproductive cells are on the same frond). P. dioica grows on intertidal, sandy rocks and is most abundant in the spring and early summer. The membranous frond is only one cell thick and is olive-green to purple-brown or blackish. This species appears to have a limited distribution in western Europe, but the genus is widespread throughout the world. All species of Porphyra are edible and are often harvested for food worldwide, especially in Japan where they are cultivated and known as nori. In the UK, wild laver is collected and made into the Welsh delicacy laverbread. into a divided blade, which is slightly rolled to form a channel with a thickened edge. Reproductive structures housed in small nodules on the blade’s surface produce a very different seaweed in the form of a thick black crust (it was originally named Petrocelis cruenta because it was thought to be an entirely different species). Spores from this crust grow back into the erect form, in a typical two-phase life history. Mastocarpus stellatus and the similar Chondrus crispus are both known as Irish moss or carrageen moss and are collected on an industrial scale on both sides of the north Atlantic to produce the gelling agent carrageenan.
DIVISION RHODOPHYTA
245
DIVISION RHODOPHYTA
Maerl
Cotton’s Seaweed
Phymatolithon calcareum DIAMETER
Kappaphycus alvarezii
Up to 23/4 in
LENGTH
(7 cm)
20 in (50 cm)
Intertidal and shallow subtidal rocks
HABITAT
Subtidal seabed sediments
HABITAT
WATER TEMPERATURE
50–86˚F (10–30˚C)
WATER TEMPERATURE
32–77˚F (0–25˚C) DISTRIBUTION Coasts of Atlantic islands, northern and western Europe, Mediterranean, and Philippines
The term “maerl” describes various species of unattached coralline seaweeds that live on seabeds. Phymatolithon calcareum forms brittle, purple-pink, branched structures that look more like small corals than seaweed. It grows as spherical nodules at sheltered sites, or as twigs or flattened medallions at more exposed sites. In places with some water movement from waves and tides, but not enough to break the maerl nodules, extensive beds can develop. Maerl is as much a habitat as a species, and both the living maerl and the maerl-derived gravel beneath it harbour many small animals. Maerl grows slowly and the beds are vulnerable to damage from bottom trawlers.
DIVISION RHODOPHYTA
Coral Weed
DISTRIBUTION Coasts of Africa, southern and eastern Asia, and Pacific islands
Formerly called Eucheuma cottonii, this is a much-branched, cylindrical red seaweed that is farmed extensively in the Philippines for extraction of carrageenan, a gelling agent similar to agar (see panel opposite). In the wild, it grows attached to rocks or lies loose in sheltered places. Like some other red seaweeds, its fronds are often shades of green and brown rather than red.
Corallina officinalis LENGTH
Up to 43/4 in
(12 cm) Rock pools and shallow subtidal rocks
HABITAT
WATER TEMPERATURE
DIVISION RHODOPHYTA
Spectacular Seaweed Drachiella spectabilis
32–77˚F (0–25˚C) DISTRIBUTION
LENGTH
Up to 21/2 in
(6 cm)
Coasts worldwide except for far north
and Antarctica
Subtidal rocks at 6–100 ft (2–30 m)
HABITAT
WATER TEMPERATURE
46–64˚F (8–18˚C) Off western coasts of Scotland, UK, Ireland, France, and Spain DISTRIBUTION
OCEAN LIFE
Coral weed belongs to a group of red seaweeds known as coralline seaweeds, which have chalky deposits in the cell walls that give them a hard structure. Coral weed fronds have rigid sections that are separated by flexible joints. The branches usually lie in one plane, forming a flat, featherlike frond, but the shape is very variable. On the open shore, the fronds are often stunted, forming a short mat a few inches high in channels and rock pools and on wave-exposed rocks. These mats often harbor small animals, and other small red seaweeds attach to the hard fronds. Subtidally, the fronds grow much longer. The color of coral weed varies from dark pink when it lives in the shade to light pink in sunny locations. When the seaweed dies, its hard white skeleton becomes part of the sand.
This colorful seaweed is rarely seen, except by divers, because it normally grows in relatively deep water and is rarely washed ashore. It also grows in shallower water within kelp forests. It has a thin, fan-shaped frond, split into wedges, that spreads out over the rock and reattaches with small rootlike structures called rhizoids.Young plants have a purple-blue iridescence, which is lost as the seaweed ages. Sexual reproduction is unknown in this species and spores are produced asexually.
246
PLANT LIFE
Green Seaweeds
frond
GREEN ALGAE LARGE ENOUGH
to be seen with the naked eye are known as green seaweeds. They are classified with the microscopic green algae, or microalgae (see p.248). True plants of the sea, they have pigments and other features in common with higher plants. They can be abundant in tropical lagoons, and proliferate seasonally on many temperate seashores. Ulva (sea lettuce) is grown for food.
DOMAIN Eucarya KINGDOM Plantae DIVISION Chlorophyta CLASSES About 8 SPECIES 5,426
Habitats
Anatomy
Green seaweeds often attach to rocks on rocky coasts, particularly in temperate and cold waters, and are ephemeral colonizers in seasonally disturbed tidal and shallow subtidal habitats. Ulva species, such as sea lettuce, dominate in high-level rock pools, or where fresh water seeps over the shore, since they can withstand changes in saltiness and temperature. The more delicate Cladophora and Bryopsis species live in rock pools or among red and brown seaweeds in the shallow subtidal zone. Green seaweeds also thrive in shallow, tropical lagoons, where species of Caulerpa, Udotea, and Halimeda are often abundant. Caulerpa species have runners (stolons), which creep through sand or cling to rock, while the bases of Udotea and Halimeda are a bulbous mass of fibers that anchor in sand. Halimeda (cactus seaweed) is heavily encrusted with calcium carbonate, which breaks up when the plant dies, contributing to the lagoon sand.
The body structure of green seaweeds lacks stems and roots. Green seaweed shapes range from threadlike (filamentous) to tubes, flat sheets, and more complex forms. Their bright green color is due to the fact that their chlorophyll is not masked by additional pigments, unlike red and brown seaweeds. Many of the features of green algae, including their types of chlorophyll, are shared by higher plants (mosses, liverworts, and vascular plants), so green seaweeds appear to be more closely related to higher plants than to red and brown seaweeds.
large, fibrous holdfast
SEAWEED BODY PARTS
Green seaweeds have a simple structure, with an erect frond and a disk-shaped or fibrous holdfast. This tropical Udotea species has calcified fronds with many branched siphons. FRAGILE FRONDS
This delicate Bryopsis plumosa has coenocytic fronds, meaning its fronds do not have the crosswalls common in other green seaweeds.
CODIUM FOREST
This mini-forest of Codium fragile is growing on shallow rocks in a sheltered bay in Scotland. The fronds are buoyant, holding the plants up to the light.
FLEXIBLE SEAWEED
Able to handle fluctuations in salinity and temperature, Ulva species thrive in this freshwater stream as it flows across the seashore.
CLASS ULVOPHYCEAE
Flaccid Green Seaweed Ulothrix flacca SIZE
Up to 4 in (10 cm)
Intertidal on various shore types
HABITAT
HABITAT
CLASS ULVOPHYCEAE
Sea Lettuce Ulva lactuca SIZE
Up to 40 in (100 cm)
Intertidal and shallow subtidal
HABITAT
WATER TEMPERATURE
32–86˚F (0–30˚C)
32–68˚F
(0–20˚C)
OCEAN LIFE
attached to its rock by a single cell called a basal cell, which may be given additional anchorage by outgrowths called rhizoids. This seaweed reproduces by releasing up to a hundred gametes, each with two flagellae, from some of the cells. In another phase of its life cycle it is a single globular cell.
DISTRIBUTION Northern Atlantic, Mediterranean, waters off South Africa, Pacific
This seaweed is made up of many unbranched green filaments, which themselves consist of strings of cells. The filaments form soft, woolly masses or flat green layers that stick to intertidal rocks. Each filament is
DISTRIBUTION
Coastal waters worldwide
Sea lettuce is common worldwide on seashores and in shallow subtidal areas, growing in a wide range of conditions and habitats. Its frond is a bright green, flat sheet, which is often split or divided, and has a wavy edge. The plant is very variable in shape and size, ranging from short, tufted plants on exposed shores to
sheets over a yard long in sheltered, shallow bays, especially where extra nutrients are available in polluted harbors. Sea lettuce reproduces by releasing gametes from some cells, and it can also spread vegetatively by regeneration of small fragments. Large fronds lying on the seabed may be full of holes made by grazing animals. It is a also popular food for humans in many parts of the world.
GREEN SEAWEEDS CLASS CLADOPHOROPHYCEAE
Giant Cladophora Cladophora mirabilis LENGTH
Up to 40 in (100 cm) HABITAT
Subtidal rocks and kelp WATER TEMPERATURE
50–59˚F (10–15˚C) DISTRIBUTION
Southern Atlantic off southwest
Africa
CLASS BRYOPSIDOPHYCEAE
Sea Grapes Caulerpa racemosa HEIGHT
Up to 12 in (30 cm) HABITAT
Shallow sand and rock WATER TEMPERATURE
59–86˚F (15–30˚C) CLASS CLADOPHOROPHYCEAE
Sailor’s Eyeball Valonia ventricosa SIZE
Up to 11/2 in (4 cm)
Rock and coral to 100 ft (30 m)
HABITAT
WATER TEMPERATURE
50–86˚F (10–30˚C) DISTRIBUTION Western Atlantic, Caribbean, Indian and Pacific oceans
CLASS BRYOPSIDOPHYCEAE
This odd seaweed looks like a dark green marble, and consists of a single large cell attached to the substrate (which is often coral rubble) by a cluster of filaments called rhizoids. Younger plants have a bluish sheen, but older ones become overgrown with encrusting coralline red seaweeds. Sailor’s eyeball has an unusual way of reproducing vegetatively: daughter cells are formed within the parent, which then degenerates, releasing the young plants in the process.
DISTRIBUTION
Warm waters worldwide
247
A giant among Cladophora species, C. mirabilis grows to 40 in (1 m) long. It is bluish green and filamentous, with many straggly side-branches. It is made up of strings of cells, but individual cells in the main axis may be 1/2 in (12 mm) long. The plant attaches using a disk made of interwoven extensions of its basal cell, and often has red algae growing on it. It has a very limited distribution in South Africa, but other species of Cladophora are common worldwide.
KILLER SEAWEED A strain of Caulerpa taxifolia that is widely used in marine aquariums is an invasive species. It is toxic to grazers, grows rapidly, and forms a dense, smothering carpet on the seabed. In 1984 it was discovered in the Mediterranean off Monaco, and has since spread rapidly along the coast, altering native marine communities.
This seaweed has creeping stolons (stems) that anchor it to rocks or in sand, and from which arise upright shoots covered with round sacs, or vesicles, hence the common name sea grapes. Each plant is a single huge cell. Old plants may become densely branched and entangled, growing to 6 ft (2 m) across. There are many varieties of sea grapes, and around 60 species of Caulerpa worldwide.
CLASS BRYOPSIDOPHYCEAE
Cactus Seaweed
Velvet Horn
Halimeda opuntia
Codium tomentosum SIZE
SIZE
Up to 10 in (25 cm)
Up to 8 in (20 cm)
Intertidal pools, shallow subtidal rocks HABITAT
HABITAT
Rock and sand
WATER TEMPERATURE
46–86˚F (8–30˚C)
WATER TEMPERATURE
68–86˚F (20–30˚C) DISTRIBUTION
Red Sea, Indian Ocean, and western
DISTRIBUTION
Coastal waters worldwide
Pacific
CLASS DASYCLADOPHYCEAE
Mermaid’s Wineglass Acetabularia acetabulum SIZE
11/4 in (3 cm)
calcium carbonate, and it terminates in a small cup. The cup is made up of fused rays that produce reproductive cysts. The cysts are released after the remainder of the plant has decayed, and they then require a period of dormancy in the dark before they begin to germinate.
HABITAT
Shallow subtidal rocks WATER TEMPERATURE
50–77˚F (10–25˚C) Eastern Atlantic off North Africa, Mediterranean, Red Sea, Indian Ocean DISTRIBUTION
This curious little green alga grows in clusters on rocks or shells covered with sand in sheltered parts of rocky coasts within its range. Although it grows to 11/2 in (3 cm), it consists of just one cell. Its calcified frond appears white because it is encrusted with
OCEAN LIFE
The heavily calcified skeletons of species of Halimeda contribute much of the calcareous sediment in the tropics. The plant consists of strings of flattened, kidney-shaped, calcified segments, linked by uncalcified, flexible joints. By day, its chloroplasts are in the outer parts of the frond; at night they withdraw deep into the plant’s skeleton. This, along with sharp crystals of aragonite, and the presence of toxic substances in the frond, protects them from nocturnal grazing.
The spongy fronds of velvet horn are made up of interwoven tubes, arranged rather like a tightly packed bottlebrush, with each tube ending in a swollen bulb. Many of these bulbs packed together make up the outside of the frond, which is usually repeatedly branched in two. Many short, fine hairs cover the seaweed, giving it a fuzzy appearance when in water. The plants are attached to rocks by a spongy holdfast. Although this seaweed is present year-round, its maximum development is in winter, and it also reproduces during the winter months.Velvet horn, like all Codium species, is often grazed by sacoglossans, small sea slugs that suck out the seaweed’s contents, but can keep the photosynthetic chloroplasts alive and use them to make sugars inside their own tissues. The chloroplasts color the sea slugs green, which helps to disguise them from predators. There are about 50 species of Codium.
248
PLANT LIFE
Green Algae
Anatomy
THESE MICROSCOPIC, MOSTLY single-celled
DOMAIN Eucarya KINGDOM Plantae
plants live in the surface layers of the ocean in CLASS Prasinophyceae immense numbers, and SPECIES 200 they form an important part of the phytoplankton (see p.212). Sometimes referred to as the “grasses of the sea,” like most plants, they produce their own food through photosynthesis. Large green algae, visible to the naked eye, are called green seaweeds and are discussed elsewhere in this book (see p.246). Microscopic algae are often termed “microalgae.” Green microalgae are frequently classified as protists. Numerous other groups of protists (see p.236) are also termed algae, and also live as phytoplankton. DIVISION Chlorophyta
Habitats With a few exceptions, marine microalgae swim and float, in countless millions, in the sunlit layers of the ocean so that photosynthesis can occur. They are more numerous in nutrient-rich waters, such as those benefiting from coastal runoff. In temperate coastal waters, green microalgae multiply rapidly each spring in response to rising nutrient and light levels, creating abundant food for zooplankton. Such population explosions, or blooms, can reduce the water clarity for weeks. Some green algae live inside the bodies of animals (see panel, right) and inside protist plankton—in the appendages (rhizopoda) of radiolarians (see p.237) and within compartments inside the dinoflagellate Noctiluca (see p.236).
HALOSPHAERA
These microalgae (shown greatly enlarged) are green with chlorophyll and bear hairlike swimming appendages called flagellae.
Marine microscopic green algae mostly belong to a class of algae called the Prasinophyceae. Each consists of a single living cell that is generally too small to be visible to the naked human eye. Even the larger species, such as members of the genera Halosphaera and Pterosperma, measure just 0.1–0.8 mm across, so appear as no more than a speck. Some green algae can swim, and beat two or more hairlike structures, called flagellae, to move through the water. Others lack flagellae and cannot propel themselves. Several groups of these plants have a two-stage life history, including both swimming and non-swimming forms. All green algae possess chloroplasts—structures GREEN BEACHES that contain the green pigment chlorophyll A few green algae and worms form symbiotic partnerships, in that plants use in which both species gain. The photosynthesis. beach-living worms ingest algae, giving them a green color. At low tide, they move up through the sand to pools on the surface, where the algae photosynthesize. In return, the worms absorb food from the algae.Vast numbers of the worms tinge beaches green.
ANIMAL–ALGA PARTNERSHIP
When young, these marine flatworms ingest green algae, which may multiply until there are 25,000 algal cells living in each worm. The adult worms obtain all their nutrition from the algae.
GREEN TIDE
Green algae grow quickly and are the first to respond in spring when nutrients become available. While grazer levels are low, the algae are free to multiply until their density turns the ocean green.
CLASS PRASINOPHYCEAE
Halosphaera viridis SIZE
20–30 micrometers (motile phase)
CLASS PRASINOPHYCEAE
Tetraselmis convolutae SIZE
10 micrometers
OCEAN LIFE
DISTRIBUTION
Northeastern Atlantic, eastern Pacific
Halosphaera viridis is a small, pearshaped cell with four swimming flagellae at one end. It reproduces by splitting in two, allowing it to reach high concentrations and from time to time some cells become small cysts whose contents divide into small disks. Each disk eventually becomes a flagellated cell that will be released into the sea. There can be hundreds of cysts per square yard in the open ocean, and they are probably a vital food source for larger zooplankton.
Northeastern Atlantic, off the western coasts of Britain and France
DISTRIBUTION
Although it can survive free-living, the tiny cells of Tetraselmis convolutae often live inside a worm host (see box, above) in a symbiotic relationship.The worm provides them with shelter and a constant environment inside its body.The worm’s light-seeking behavior gives the algae ideal conditions for photosynthesis, which in turn provides both algae and worm with nutrients and energy.
MOSSES
Mosses
249
Anatomy MOSSES ARE LOW-GROWING
plants that thrive in damp habitats on land, KINGDOM Plantae where they may carpet the ground or DIVISION Bryophyta rocks. They dislike salty environments SPECIES 13, 365 and only a few species manage to live in the intertidal zone of coasts, mainly in cooler climates. A much wider variety of mosses can be found slightly farther inland, away from the direct effects of sea spray but within range of moisture-laden sea mists. DOMAIN Eucarya
Most mosses have a recognizable structure of stems and leaves, which, as in other plants, gather sunlight and perform photosynthesis. However, unlike flowering plants (see p.250), they do not have woody tissues for support, and they also lack the conducting tissues that transport water and nutrients. Mosses have a very thin outer layer of cells, or cuticle, that can absorb (and lose) water, nutrients, and gases over their entire surface. Their “roots” are simple strands called rhizoids, which anchor the plant to its growing surface. Mosses SPORE PRODUCTION reproduce sexually by means of Mosses have low-growing leaves, but sprout wind-blown spores, or asexually taller structures with bulbous tips called capsules, from which spores are released. by spreading across the ground.
Habitats Mosses generally prefer moist, shady places and are most numerous in the cooler and damper climates of temperate regions. This is because they lack the thick cuticle that enables other types of plant to retain moisture. Without the protection of this skinlike surface, mosses soon shrivel up in dry conditions. However, some mosses have an amazing capacity to recover quickly when wetted after a long period of drought. A few species grow in salt marshes or among the lichens at the top of rocky shores; on sheltered coasts, where there is little salt spray, they may live only just above the high tide level. Sand-dune mosses grow rapidly to keep pace with accumulating sand, and blown fragments of moss can colonize new areas of dunes. Many more moss species grow on sea cliffs and in damp gullies away from the intertidal zone. SYNTRICHIA RURALIFORMIS
This moss grows in coastal sand dunes. Its leaves curl up when dry (left of picture) but unfurl a few minutes after wetting (on right).
CLASS BRYOPSIDA
CLASS BRYOPSIDA
CLASS BRYOPSIDA
Golden Dune Moss
Salt Marsh Moss
Seaside Moss
Syntrichia ruraliformis
Hennediella heimii
Schistidium maritimum
SIZE
1/ –11/ 2 2
in (1–4 cm)
SIZE
Yellow-green to orange-brown cushions and carpets
FORM
HABITAT
1/ 8
FORM
in (3 mm)
Single green
plants Salt marshes, other coastal areas
HABITAT
Mobile dunes
SIZE
1 in (2 cm)
Dark blackish green, compact cushions
FORM
Hard, acidic rocks, salt marshes
HABITAT
DISTRIBUTION
Patchy distribution on temperate and cool waters worldwide
DISTRIBUTION Western and eastern coasts of North America, coasts of western Europe
This is one of the first mosses to colonize mobile dunes. It often forms extensive colonies that cover many square yards of sand, giving the sand a golden tinge. Its leaves are covered by hundreds of small papillae that enable swift absorption of water. The leaves gradually taper into long white hair points. This moss is able to establish new plants from fragments dispersed by the wind.
This tiny moss is a halophyte, meaning it is adapted to growing in highly saline conditions. It is rarely found growing inland. One of the few mosses that may be regularly found in salt marshes, it grows on patches of bare ground between the other vegetation in the upper parts of the salt marsh. It also grows in various other coastal habitats, including the banks of creeks, behind sea walls, and on footpaths. Although small, the plants may be abundant and may appear conspicuous from a distance, due to their prolific number of stout, dark, rusty-brown capsules, which are borne on short stalks less than 1/2 in (1 cm) tall. These have a little cap with a long point, which lifts to allow spores to escape, but remains attached to the capsule by a central stalk. The Salt marsh Moss has a wide distribution in colder climates.
This moss grows as small, dark green cushions on hard, acidic rocks, with seashore lichens, just above high-tide mark. It also occurs in salt marshes. It is often soaked by salt spray and occasionally covered by
CLASS BRYOPSIDA
Southern Beach Moss Muelleriella crassifolia SIZE
1–5 in (2–13 cm) FORM
Black cushions and mats HABITAT
Rocks DISTRIBUTION Southern tip of South America, islands in the Southern Ocean
This moss is the southern version of seaside moss (see above), growing on coastal rocks in the usually lichen-dominated splash zone, where it is often inundated by the sea in stormy weather. It grows in southern Chile and on subantarctic islands, where it can become dominant. On Heard Island, for example, a salt spray community of plants found on exposed coastal lava rock, at elevations of less than 16 ft (5 m), is dominated by southern beach moss, which has also colonized derelict buildings.
OCEAN LIFE
DISTRIBUTION Eastern Pacific, northwestern Atlantic, Mediterranean
the highest tides. Seaside moss appears to be a true halophyte, functioning normally even after immersion in sea water for a few days, and growing only in saline conditions; in Britain, it is found no farther than 1,300 ft (400 m) from the sea. Its leaves curl when dry. In winter, it produces small brown capsules on short stalks.
250
PLANT LIFE
Flowering Plants PLANTS CONQUERED LAND,
and then land-based flowering plants KINGDOM Plantae grew to be among the most DIVISION Trachaeophyta abundant and diverse life-forms CLASS Angiospermae on Earth. However, relatively SPECIES 260,000 few have adapted to the poor soil, salt spray, and drying wind of coastal dunes and cliffs. These few include some fascinating plants found nowhere else. Few flowering plants have returned to the sea: salt-marsh plants and mangroves get wet at high tide, but only the seagrasses live fully submerged. DOMAIN Eucarya
Anatomy
OCEAN LIFE
Flowering plants, technically called angiosperms, uniquely possess fruit and flowers, unlike mosses, seaweeds, and other algae. They are adapted to life in air, absorbing fresh water through their roots. If they take salt water into their vascular system, water from their own cell sap is attracted to the more concentrated salts and sucked out by osmosis. This is fatal to cells, but mangroves cope by excreting the salt, while succulents partition it within their cells. Seagrasses have fully adapted by matching their cells’ salt concentration to that of seawater. Most seagrasses have a similar form, with thin, grasslike blades that allow easy exchange of nutrients and gases. Mangroves grow in mud that lacks oxygen by growing aerial roots to waterproof assist gas exchange in their underground roots. Many seed case angiosperm seeds are killed by seawater, but those of the coconut can stay viable at sea for long periods inside a waterproof case. Seagrasses are germinating water-pollinated, and to plant GERMINATING SEED increase the chances of a This coconut is a fruit—a defining pollen grain catching onto characteristic of flowering plants. The a female stigma, pollen is coconut has the marine adaptations of buoyancy and a waterproof case. released as a sticky string.
Seawater Plants Seagrasses are monocotyledons (the group of flowering plants with narrow, straplike leaves), but are not true grasses, and they do not share a single evolutionary origin. There are 59 species in 5 families, although the Ruppiaceae, living mainly in brackish water, is not always accepted as a seagrass family. Salt-marsh plants are mainly small and herbaceous, with early colonizers including the salt-excreting cord grass and small succulents, such as common glasswort. Further salt-tolerant flowering plants grow farther up the shore in established salt marshes, forming a dense, grassy turf. Salt marshes (see p.124) form in cooler climates, and are replaced in tropical seas by mangroves—trees with characteristic aerial roots. There are 16 families and 54 species of mangroves. Like seagrasses, they do not have a single origin, so the mangrove habit evolved separately, several times. EELGRASS MANGROVES SUBMERGED
When the tide is in, mangroves form a mini-jungle of arching roots where small fish hide.
Seagrasses, such as this eelgrass in a Scottish sea loch, can be found from the cold waters of Alaska to tropical seas.
Coastal plants A greater variety of flowering plants can grow above the high-water mark. Salt-tolerant grasses are important constituents of the upper parts of salt marshes, and at the seaward edge of sand-dune systems, grasses are often the first to stabilize the shifting sand. In sand and sheltered gravel at the top of the shore, a few deep-rooted plants grow. A much wider variety of flowers and a few mosses colonize sand dunes and slacks just inland from the coast. Here they are subjected to salt spray but never inundated by tides. Nitrogen fixers thrive in these poor, sandy soils. In warmer climates, annuals bloom like desert flowers after seasonal rains. On cliff-tops, plants may be fertilized by sea-bird guano, stimulating lush growth.
DUNE-SLACK FLORA COASTAL FLOWERS
The beautiful pink flower heads of sea pink transform rocky seashores and salt marshes in late spring. The sea pink’s compact cushions resist wind and cold.
Here, in dune slacks behind a beach in the Canary Islands, annual plants bloom for a short period after rain. They flower and produce seeds quickly before drying up in the summer sun.
FLOWERING PLANTS ORDER NAJADALES
ORDER HYDROCHARITALES
Marram grass is a tall, spiky grass that plays a key role in binding coastal sand and building sand dunes. Its underground stems (rhizomes) spread
through loose sand, and upright shoots develop regularly along their length. When the tangle of stems and leaves impede onshore breezes, sand carried in the wind is deposited. Progressively, the sand builds up, the stems grow up through the sand, and a sand dune is formed. In dry weather, the leaves curl into a tube. The underside of the leaf then forms the outer surface and its waxy coating helps to reduce water loss from the plant. Marram grass is widely planted to stabilize eroded dunes, and has been introduced for this purpose to North America (where it is known as European beach grass), Chile, South Africa, Australia, and New Zealand.
thick, waxy skin. It is able to prevent the salt absorbed through its roots from doing any damage by locking it away in vacuoles (small cavities) within its cells. The plant stores water inside its succulent stems, hence its cactuslike shape. For centuries, glasswort was gathered and burnt to
produce an ash rich in soda (impure sodium carbonate). The ash was then baked and fused with sand to make crude glass—hence its common name. Glasswort can also be eaten boiled or pickled in vinegar. It has a mild, salty flavor, and is also known as poor man’s asparagus.
ORDER POALES
Neptune Grass
Paddle Weed
Marram Grass
Posidonia oceanica
Halophilia ovalis
Ammophila arenaria
DISTRIBUTION
TYPE
TYPE
TYPE
Perennial
Perennial
Perennial
HEIGHT
HEIGHT
HEIGHT
12 in (30 cm)
21/2 in (6 cm)
11/2–4ft (0.5–1.2 m)
HABITAT
HABITAT
HABITAT
Rocks and sand
Sand
Coastal sand dunes
Mediterranean
Coasts of Florida, USA, East Africa, Southeast Asia, Australia, and Pacific islands
DISTRIBUTION
Neptune grass (also known as Mediterranean tapeweed) forms meadows from shallow water to a depth of 150 ft (45 m) in the clearest waters. It grows on both rock and sand, has a tough, fibrous base, and persistent rhizomes (stems) that grow both horizontally and vertically. These build up into a structure known as “matte,” which can be several meters high and thousands of years old. Around the island of Ischia, Italy, more than 800 species have been associated with Neptune grass beds.
DISTRIBUTION Western Europe and Mediterranean (natural occurrence); introduced elsewhere
251
Members of the genus Halophila look quite unlike other seagrasses, having small, oval leaves that are borne on a thin leaf stalk. As its scientific name indicates, paddle weed is particularly tolerant of high salinities (halophila means “salt-loving”). Pollination takes place underwater, and the tiny, oval pollen grains are released in chains, which assemble into rafts like floating feathers. This is thought to increase the chances of pollination of a female flower. Despite its small size, paddle weed is an important food for the dugong (see p.419). An adult can eat more than 88 lb (40 kg) of it a day.
ORDER CARYOPHYLLALES
Common Glasswort Salicornia europaea Annual
4–12 in (10–30 cm)
HEIGHT
Coastal mudflats and salt marshes
HABITAT
DISTRIBUTION Coasts of western and eastern North America, western Europe, and Mediterranean
OCEAN LIFE
TYPE
Glasswort, also known as marsh samphire, is an early colonizer of the lower levels of salt marshes and mudflats, where plants are inundated twice a day by the tide. It is a small, cactuslike plant with bright green stems that later turn red. The tiny flowers and scale-like leaves are sunk into depressions in the fleshy stem. Glasswort is protected externally from salt water and moisture loss by a
252
PLANT LIFE ORDER PLUMBAGINALES
Common Sea Lavender Limonium vulgare
in Wales, while others are only found in parts of Sicily or Corsica.Varieties of sea lavender, often called statice, are grown commercially as “everlasting” flowers. The colored, papery “flowers” are actually what remains after the true flowers have fallen.
ORDER PAPAVERALES
Yellow Horned-poppy Glaucium flavum
Perennial
Biennial or perennial
HEIGHT
HEIGHT
TYPE
TYPE
20–36 in (50–90 cm)
8–20 in (20–50 cm)
HABITAT Shingle, sometimes sand
HABITAT
Muddy salt marshes DISTRIBUTION Coasts of western Europe, Mediterranean, Black Sea, and Red Sea
DISTRIBUTION
Coasts of western Europe, Mediterranean, and Black Sea
This showy plant, which flowers in late summer, often forms dense colonies in salt marshes, particularly along the sides of muddy creeks. Several closely related species of sea lavender are highly localized in their distribution. For example, two species are confined to two rocky peninsulas
Also known as yellowhorn poppy, this plant has leaves covered in a waxy coating to protect it from salt spray and reduce water loss. Its taproot penetrates deep into shingle in search of water beneath. It blooms through most of the summer, producing flowers that are up to 31/2 in (9 cm) across.
ORDER MYRTALES
ORDER POLEMONIALES
Beach Morning-glory
Pacific Stilt-mangrove
Ipomoea imperati
Rhizophora stylosa TYPE
Perennial
LENGTH
HABIT
Woody perennial
Commonly 5–8 m (16–26 ft), but can be up to 40m (130 ft)
Up to 16 ft
HEIGHT
(5 m) Coastal beaches and grasslands
HABITAT
HABITAT
Intertidal
mudflats DISTRIBUTION Widespread on many coasts and islands with tropical or warm temperate climates
This pioneer plant helps to stabilize coastal sands, creating a habitat into which other species move. It can endure low nutrient levels, high soil temperatures, abrasion and burial by blown sand, and occasional frosts, but not hurricanes, according to studies in Texas. It also grows occasionally on disturbed ground inland. Beach morning-glory is recorded on six continents and many isolated islands.
PROFILE CAPPARALES
Scurvy-grass Cochlearia officinalis HABIT Biennial or perennial HEIGHT 4–16 in (10–40 cm)
Coastal rocks and salt marshes
HABITAT
OCEAN LIFE
DISTRIBUTION Coasts of northern Europe and Asia and northern North America
The thick, fleshy leaves of this coastal plant help it to store water in an environment where fresh water soon drains away (scurvy-grass plants found on mountains have thinner leaves and may belong to a different species). Scurvy-grass leaves are rich in vitamin C. They were once eaten, or pulped and drunk, to prevent scurvy—a disease caused by vitamin C deficiency to which sailors were prone (“grass” is Old English for any green plant).
Coasts of northern Australia, Southeast Asia, and South Pacific islands
DISTRIBUTION
The aerial roots of Pacific stiltmangroves arch down from the main trunk, with secondary roots coming off the primary ones before they reach the ground to form a tangle of roots growing in all directions. When the tide is in, these form a sheltered refuge for many small fish. This species of mangrove can tolerate a wide range of soils, but thrives best in the fine, muddy sediments of river estuaries. Its roots absorb water selectively, so much of the damaging salt is not taken up, but it still has to excrete some salt through the leaves.
FLOWERING PLANTS ORDER CARYOPHYLLALES
Grand Devil’s-claw Pisonia grandis TYPE
Woody perennial
46–98 ft (14–30 m)
HEIGHT
Coastal and island forests
HABITAT
DISTRIBUTION Coasts and islands in Indian Ocean, Southeast Asia, and South Pacific
ORDER ARECALES
The grand devil’s-claw is typically found on small tropical islands and its distribution is associated with sea bird colonies. It can grow as tall as 98 ft (30 m), the trunk can be up to 61/2 ft (2 m) in diameter, and it is often the dominant tree in coastal forests that are undisturbed by humans. The trees provide nesting and roosting sites for many species of sea bird, whose guano is an important fertilizer on isolated islands. The branches break easily, and can root in the ground.
BIRD-KILLING TREE The seeds of grand devil’s-claw are produced in clusters of 50–200 and exude a resin that makes them extremely sticky. They attach to the feathers of sea birds and may subsequently be flown to remote islands. This is an effective means of dispersal, but the seeds are so sticky that small birds often become completely entangled and die.
253
ORDER CASUARINALES
Casuarina Casuarina equisetifolia TYPE
Woody perennial
66–98 ft (20–30 m) HEIGHT
Coastal and island forests
HABITAT
DISTRIBUTION Southeast Asia, eastern Australia, and islands in southeast Pacific
Casuarina has many common names, including Beach she-oak, beefwood, ironwood, and Australian pine. It is typically found at sea level, but also grows inland to 2,600 ft (800 m). Casuarina is fast-growing, reaching a height of 65 ft (20 m) in 12 years. It is drought-tolerant, and can grow in poor soils because it can fix nitrogen in nodules on its roots. Its wood is very hard and is used as a building material and as firewood. The bark is widely used in traditional medicines.
COCONUT TREE
Coconut Palm Cocos nucifera HABIT
Woody perennial
The coconut palm can live as long as 100 years, a mature tree producing 50–80 coconuts a year. The trunk is ringed with annual scars left by fallen leafbases.
66–72 ft (20–22 m)
HEIGHT
Coastal rocky, sandy, and, coralline soils
HABITAT
DISTRIBUTION
Tropical and subtropical coasts
worldwide
The coconut palm was once the mainstay of life on Pacific islands. It provided food, drink, fuel, medicine, timber, mats, domestic utensils, and thatching for roofs. It remains an important subsistence crop on many Pacific islands today. Its original habitat was sandy coasts around the Indo-Malayan region, but it now is found over a much wider area, assisted by its natural dispersal mechanism, and deliberate planting by humans. The fibrous husk of the coconut fruit is a flotation aid that enables the seeds to be carried vast distances by ocean waves and currents. The coconut palm cannot develop viable fruits outside of the tropics and subtropics. COCONUT FRUIT
The fruit of the Coconut Palm weighs 2¼–4½ lb (1–2 kg). It contains one seed, which is rich in food reserves and is part solid (flesh) and part liquid (coconut milk).
fibrous husk
OCEAN LIFE
edible flesh
254
FUNGI
Fungi FUNGI FORM A GREAT KINGDOM OF SINGLE-CELLED
and filamentous life-forms, including yeasts and moulds. Some organise their filaments KINGDOM Fungi into complex fruiting structures, such as mushrooms. Truly marine PHYLA 5 fungi are rare, but a few fungus-like organisms survive within a slime SPECIES 46,574 covering, avoiding contact with salt water. Fungi are abundant on shorelines, but only in close association with certain algae. Alga and fungus grow in partnership in a kind of symbiotic, compound organism called a lichen. Lichens proliferate in the hostile, wave-splashed zone of bare rock just above high tide. DOMAIN Eucarya
Anatomy A lichen’s body (thallus) is composed mainly of fungal filaments called hyphae. The cells of the fungus’s algal partner are restricted to a thin layer below the surface, where they cannot dry out. Lichens grow in one of four ways: bushy (fruticose); leaf-like (foliose); tightly clustered (squamulose); or lying flat (crustose). Marine fungus-like organisms, such as slime nets (labyrinthulids) and thraustochytrids, are microscopic, usually transparent, and encased in a network of slimy threads. The cells move up and down within the threads and react positively towards food. They are increasingly recognized as protists, however, rather than fungi. LICHEN COMPOSITION
This false-colour micrograph of a lichen (below) shows the smooth surface of the thallus, to the left, and fungal hyphae, to the right.
ENCASED IN SLIME
This thraustochytrid (above) is a fungus-like organism that lives as a parasite within certain bivalves. Its slime net forms a complete cover.
OCEAN LIFE
Habitats Most lichens require alternating dry and wet periods, but marine lichens can withstand continuous drought or dampness. On most rocky shores, yellow and grey lichens dominate surfaces splashed by waves at high tide (the splash zone). They endure both the drying Sun and wind, and the salt spray of the sea. Below, in the tidal zone, the brightly coloured lichens give way to black encrusting lichen, such as Verrucaria maura, which covers the bedrock and any large, stable boulders. Verrucaria serpuloides lives yet further down the shore and is the only lichen to survive permanent immersion in sea water. Slime nets can live in the sea because they are protected from the dehydrating effects of salt water by slime, or because they live as parasites within seagrasses, green algae, or clams. BELOW THE SPLASH ZONE
Some lichens, such as this crustose black Verrucaria, live below the splash zone, and may be surrounded by seaweeds.
LICHEN ENCRUSTATION
Fungi thrive on the coast if they grow in association with algae, in an intimate symbiosis called lichen. Here, encrusting and foliose lichens cover sandstone cliffs in the Shetland Isles, Scotland.
255 PHYLUM ASCOMYCOTA
Sea Ivory
PHYLUM ASCOMYCOTA
Yellow Splash Lichen
Ramalina siliquosa
Xanthoria parietina
LENGTH (BRANCHES)
WIDTH
1–4 in (2–10 cm)
Up to 4 in (10 cm) HABITAT
HABITAT
Splash zone; favors surfaces high in nitrogenous compounds
Hard siliceous rocks above the splash zone DISTRIBUTION Northeast and southwest Atlantic, coasts of Japan and New Zealand
DISTRIBUTION
Temperate Atlantic, Gulf of Mexico, Indian and Pacific oceans
Nutrient-poor siliceous rocks are the favorite habitat of gray lichens, such as sea ivory. This lichen is usually gray-green in color, with a brittle, bushlike (fruticose) structure and disk-shaped fruiting bodies, called apothecia, at its branch tips. Sea ivory cannot withstand being trampled or extensively grazed, and so it grows best on vertical rock faces, to which it sticks by a single basal attachment.
On most rocky shores, different species of lichen have a marked vertical territory related to their tolerance of salt exposure. The yellow splash lichen is found in the splash zone and forms a bright orange band across the shore, with gray lichens above it and black lichens below. It has a leaflike (foliose) form, with slow-growing, leafy lobes held more or less parallel to the rock on which it lives. Usually bright orange in color, it tends to become greener if in shade. Lichens are widely used to monitor air pollution because they simply disappear when conditions deteriorate. The yellow splash lichen is particularly sensitive to sulfur dioxide, a by-product of industrial processes and of burning fossil fuels.
PHYLUM ASCOMYCOTA
Black Tar Lichen PHYLUM ASCOMYCOTA
Verrucaria maura THICKNESS
Black Shields
1/32
in (1 mm)
HABITAT
Tephromela atra
Intertidal WIDTH
Up to 4 in (10 cm) HABITAT
DISTRIBUTION
In and above the splash zone
Ocean, Japan
Temperate and polar coasts, Indian
This smooth, black, crustose lichen covers large areas of bedrock or stable boulders in a thin layer, making them appear as though they have been covered with dull black paint. Many types of lichen accumulate heavy metals, and the black tar lichen is no exception, having been found to have levels of iron that are about 2.5 million times more concentrated than the surrounding seawater. That may be an adaptation to deter grazers, such as gastropods, from eating it.
PHYLUM ASCOMYCOTA
Gray Lichen Pyrenocollema halodytes SIZE
Not recorded HABITAT
Upper shore on rocks and on shells of some sedentary invertebrates DISTRIBUTION
Temperate northeast and southwest
Atlantic
Seen on hard, calcareous rocks, where it forms small, black-brown patches, gray lichen is unusual in being an association of three organisms— a fungus, a cyanobacteria, and an alga. The fungus anchors the lichen to the rock; the cyanobacteria and the alga contain chlorophyll and make food by photosynthesis. The cyanobacteria can also utilize nitrogen, a process that uses a lot of energy, and this comes from the sugar made during photosynthesis.
Polar coasts, coast of California, US, Gulf of Mexico, Mediterranean, Indian Ocean DISTRIBUTION
Crustose lichens such as black shields, which form a crust over the rock, attach themselves so firmly using fungal filaments that they cannot be easily removed from it. Over time, these anchoring filaments break down the rock as they alternately shrink when dry and swell when moist. Black shields is a thick, gray lichen with a rough, often cracked, surface from which project a number of characteristic black fruiting bodies. PHYLUM ASCOMYCOTA
Black Tufted Lichen Lichina pygmaea To 1/2 in (1.5 cm) HABITAT
Lower littoral fringe to middle shore, regularly covered by the tide Northeast Atlantic from Norway to northwest Africa
DISTRIBUTION
OCEAN LIFE
WIDTH (LOBES)
Typically found on exposed sunny rock faces, this lichen looks rather like a seaweed, being fruticose (bushlike) in form with branching, brownish black, flattened lobes. Its fruiting bodies form in small swellings at its branch tips. It is often seen growing in association with barnacles but does not tolerate algal (seaweed) growth. Its compact growth and rigid branches provide a refuge for several mollusks, particularly Lasaea rubra, a small, pink-shelled gastropod. All Lichina species are limited to coastal habitats.
256
ANIMAL LIFE
Animal Life ANIMAL LIFE FIRST APPEARED IN THE OCEAN
over one billion years ago. It has since diversified into a vast KINGDOM Animalia array of different organisms. The range of scale among PHYLA About 30 marine animals is immense: the smallest invertebrates SPECIES Over 1.5 million are over half a million times smaller than the largest whales. Despite this huge disparity, animals all share two key features. First, they are heterotrophs, meaning they obtain energy from food. Second, they are multicellular, which distinguishes them from single-celled life forms. DOMAIN Eucarya
Marine Animal Diversity
CHANGING SHAPE
Most invertebrates change shape as they develop. Feather stars start as drifting larvae, which eventually attach themselves to corals or rock before changing into swimming adults.
Animals are classified into 30 or more major groups (phyla), all of which include at least some marine animals. Twenty-nine of these phyla are composed of animals without backbones (invertebrates), each phylum representing a completely different body plan. Only one phylum, the chordates, contains animals with backbones (vertebrates). In salt water, vertebrates include fish, reptiles, birds, and mammals— animals that are often described as the dominant forms of ocean life. However, in terms of abundance and diversity, invertebrates have a stronger claim to this title. Invertebrates exist in all ocean VERTEBRATE habitats and outnumber marine Active predators, such vertebrates by a million to one. as this barracuda, need sharp senses and rapid They include an array of fixed reactions to catch prey. (sessile) animals, such as corals and Unlike invertebrates, they sponges. They also form most of the have fast-acting nerves zooplankton, a drifting community and well-developed of tiny animals and animal-larvae. brains.
INVERTEBRATE
This yellow tube sponge, from the sea off Belize, is a typical sessile invertebrate. Instead of moving to find food, it filters out particles of food by pumping water through its pores.
Support and Buoyancy On land, most animals have hard skeletons to counteract gravity’s pull. Life is different in the sea, because water is denser than air. It buoys up soft-bodied animals, such as jellyfish, enabling them to grow large. They use internal pressure to keep their shape, the same principle that works in balloons. Animals with hard body parts, such as fish and mollusks, are often denser than water, and would naturally sink. To combat this, many have a buoyancy device. Bony BUBBLE RAFT The violet sea snail fishes have an adjustable gas-filled swim bladder, stays afloat by producing while squid have an internal float made of chalky bubbles of mucus. material, containing many gas-filled spaces. Some The mucus slowly surface dwellers, such as the violet sea snail, have hardens, forming a permanent raft. gas-filled floats that prevent them from sinking.
OCEAN LIFE
Groups and Individuals Among marine animals, there is a social spectrum from species that live on their own to those that form permanent groups. The whale shark is a typical solitary species, spending its entire life on its own apart from when it breeds. It can do this because its huge size means it has few natural predators. Smaller fish often form shoals, which reduce each fish’s chances of being singled out for attack. Many invertebrates, from corals to tunicates, live in permanent groups, known as colonies. In most coral colonies, the individual animals, or polyps, are anatomically identical and function as independent SAFETY IN units, even though they are joined. NUMBERS Crowded together Other animal colonies, such as the in a ball, gregarious Portuguese man-of-war, are made striped catfish of individuals with distinct forms. (right) make a Each form carries out a different confusing target for predators. task, like parts of a single animal.
COLONY ON THE MOVE
LONE GIANT
A diver films a pyrosome colony in the sea off Florida. It consists of thousands of tiny soft-bodied animals called tunicates, joined together to form a tube.
The whale shark (below) is a solitary species with a pantropical range. It only congregates in particular regions during the breeding season.
ANIMAL LIFE
257
SINGLE PARENT
Reproduction Animals reproduce in two ways. In asexual reproduction, which occurs in many marine animals from flatworms to sea anemones, a single parent divides in two, or grows (buds off) parts that become independent. In sexual reproduction, the eggs of one parent are fertilized by the sperm of another. Sessile animals, such as corals and clams, usually breed sexually by shedding their eggs and sperm into the water, leaving them to meet by chance. In some fish, all mammals, and birds, fertilization is internal, which means that the two parents have to mate. Marine animals vary greatly in reproductive potential. Most whales have a single calf each time they breed, but an ocean sunfish can produce over 300 million eggs a year. COURTSHIP
This sea anemone is budding off young that will eventually take up life on their own. Asexual reproduction is quick and simple, but it does not produce genetic variation, making it more difficult for a species to adapt to change.
SEA SYMPHONY
A wrasse feeds among coral in the Red Sea. Coral reefs contain the greatest diversity of animal life in the oceans, and are one of the few habitats that are actually created by animals.
Two waved albatrosses display to each other in the Galápagos Islands. Complex courtship rituals like this ensure that each parent finds a partner of the right species and the right sex, and they cement the bond once breeding begins.
OCEAN LIFE
258
ANIMAL LIFE
Sponges
osculum
Anatomy THIS ABUNDANT AND
diverse group DOMAIN Eucarya of often colorful invertebrates lives KINGDOM Animalia permanently attached to the sea floor. PHYLUM Porifera Naturalists once thought they were CLASSES 4 plants, but they are now known to SPECIES About 8,700 be very simple animals with no close relatives. Sponges live by drawing water into their bodies through tiny holes called pores, filtering it for food and oxygen and pushing it out again. Many species are found on coral reefs or rocks, and a few live in fresh water.
The body plan of a sponge is based on a system of water canals lined with special cells known as collar cells. Collar cells are unique to sponges. They draw water into the sponge through pores, by each beating a long, whiplike flagellum. A ring of tiny tentacles around the base of the flagellum traps food particles, and the water and waste material then flows out of the sponge through larger openings. Rigidity is provided by a skeleton made up of tiny splinters (spicules) of silicon dioxide or calcium carbonate scattered throughout the body.
Habitats Most sponges need a hard surface for attachment, but some can live in soft sediment; a few species are able to bore into rocks and shells. Sponges are common on rocky reefs, shipwrecks, and coral reefs in a wide range of temperatures and depths. The largest populations occur where there are strong tidal currents, which bring extra food. Animals such as crabs and worms sometimes live inside sponges, but little manages to settle and grow on their surface. This is because sponges produce chemicals to discourage predators.
central cavity collar cell flagellum spicule
pore
BODY SECTION
A sponge has specialized cells, but no organs. Water enters the sponge through hollow pore cells and exits via larger openings called osculae.
CHANGING SHAPE
Many sponges grow different shapes in different habitats. This sponge develops fingers in strong currents (above), but has an encrusting form (right) when it grows in wave-exposed sites.
VARIETY OF FORM
Sponges come in many forms, including tubes, spheres, and threadlike shapes. Pictured are a brown tube sponge and an irregular deep red sponge.
CLASS DEMOSPONGIAE
Barrel Sponge Xestospongia testudinaria HEIGHT
Up to 6 ft (2 m) DEPTH
6–165 ft (2–50 m) HABITAT
Coral reefs DISTRIBUTION
Pacific
CLASS HEXACTINELLIDA
OCEAN LIFE
Reef-forming Sponge Heterochone calyx HEIGHT Up to 5 ft (1.5 m) DEPTH 300–800 ft (100–250 m) HABITAT
Deep hard
seabed DISTRIBUTION
Deep cold waters of north Pacific
The reef-forming sponge not only looks like a delicate glass vase, but its skeleton spicules are made from the same material as glass, silica. Each spicule has six rays, hence the scientific name of its class, Hexactinellida. Many glass sponges grow very large—off Canada’s British Columbian coast, the reef-forming sponge forms huge mounds nearly 65 ft (20 m) high spread over several miles. Other members of their class also contribute to these reefs, which may have started forming nearly 9,000 years ago. Like coral reefs, sponge reefs provide a home for many other animals.
Tropical waters of western
These gigantic sponges grow large enough to fit a person inside. Their hard surface is deeply ridged, but their rim is thin and delicate. The barrel sponge belongs to the Demospongiae, the largest class of sponges, containing about 95 percent of sponge species. The skeleton of sponges in this class is made from both scattered spicules of silica and organic collagen called spongin. An almost identical barrel sponge, Xestospongia muta, occurs in the Caribbean.
259 CLASS HOMOSCLEROMORPHA
Flesh Sponge Oscarella lobularis About 0.3 in
HEIGHT
(1 cm) HABITAT
Sublittoral rock
DISTRIBUTION
CLASS DEMOSPONGIAE
Breadcrumb Sponge Halichondria panicea To more than 12 in (30 cm)
WIDTH
Shore to sublittoral zone
DEPTH
HABITAT
Hard surfaces
DISTRIBUTION Temperate coastal waters of northeastern Atlantic and Mediterranean
The appearance of this soft encrusting sponge varies from thin sheets to thick crusts and large lumps. On waveexposed shores, it usually grows under ledges as a thin, green crust, its osculae opening at the tops of small mounds. Its green color is produced by photosynthetic pigments in symbiotic algae in the sponge’s tissues. In deeper, shaded waters, the sponge is usually a creamy yellow. In waters with strong currents, this sponge may cover large rocky areas and kelp stems.
Mediterranean and south to Senegal
The blue color of this species is unusual among sponges but this species can also be green, violet, or brown. It grows as irregular lobules that look and feel smooth and soft because it has no spicules. It doesn’t have many other skeletal fibers either and collapses when out of water. This genus of sponges, along with six others, have recently been separated out from the demosponges and placed in their own class. A similar yellow sponge that occurs around the UK is called by the same name but the two forms are thought to be different species.
CLASS DEMOSPONGIAE
Tube Sponge Haliclona fascigera HEIGHT
Up to 3 ft (1 m) DEPTH
Below 33 ft (10 m) HABITAT
Coral reefs DISTRIBUTION Tropical reef waters of western Pacific; likely to be more widespread than shown
The elegant, tubular branches of this beautiful sponge are easily torn, and so it occurs only on deeper reef slopes, where wave action is minimal.
CLASS DEMOSPONGIAE
CLASS DEMOSPONGIAE
Mediterranean Bath Sponge
Coralline Sponge Vaceletia ospreyensis
Spongia officinalis
SIZE
Not recorded
WIDTH
DEPTH
Up to 14 in (35 cm)
At least 65 ft (20 m)
DEPTH
HABITAT
3–165 ft (1–50 m)
Dark reef caves
HABITAT
DISTRIBUTION Not fully known, but includes tropical waters of western Pacific
Rocks DISTRIBUTION
It sometimes grows as a single tube, but it is more often seen as bunches of tubes joined at the base. The tips of the tubes are translucent and slightly rolled in. The color of this sponge is usually pinkish violet, although some specimens are pinkish blue. When this sponge releases sperm, it resembles smoking chimneys. The taxonomic status of this species and its relationship to other species in the same family has not been fully determined, and it is listed under various names in different sources. Such uncertainties are not unusual in the study of sponges and mean that the exact distribution of this and many other species is yet to be established.
Mediterranean, especially the
eastern part
CLASS CALCAREA
Lemon Sponge Leucetta chagosensis WIDTH Up to 8 in (20 cm) DEPTH
Shallow
Steep coral reef and rock slopes HABITAT
DISTRIBUTION
Tropical reef waters of western
Pacific
The lemon sponge is a beautiful, bright yellow color and is easy to spot underwater. It grows in the form of sacs, which may have an irregular,
OCEAN LIFE
The Mediterranean bath sponge, as its name suggests, is collected and processed for use as a bath sponge. It grows as rounded cushions and mounds, and is usually dull gray to black outside but yellowish white inside. It can be used as a sponge because it has no sharp skeletal spicules, just a network of tough fibers made from an elastic material called spongin. Huge numbers were once harvested, but today they are rare.
lobed shape. Each sac has a large opening—the osculum—through which used water flows out of the sponge. Through the osculum, entrances to the water-intake channels that run throughout the sponge can be seen. The lemon sponge belongs to a small class of sponges in which the mineral skeleton is composed entirely of calcium carbonate spicules, most of which have three or four rays. The densely packed spicules give the sponge a solid texture. Like all sponges, this sponge is hermaphroditic. It incubates its eggs inside and releases them as live larvae through the osculum. Each larva is a hollow ball of cells with flagellae for swimming.
Vaceletia ospreyensis is a living member of the coralline sponges group, most of which are known only from fossils. Coralline sponges have a massive skeleton made of calcium carbonate, as well as silica spicules and organic fibers. They were the dominant reef-building organisms before the stony corals of modern reefs evolved. Once given a separate class (the Sclerospongiae), they are now accepted as part of the Demospongiae.
260
ANIMAL LIFE
Cnidarians
HUMAN IMPACT
CORAL TRINKETS
THIS ANCIENT GROUP OF AQUATIC ANIMALS
emerged in Precambrian times, about 600 million years ago. It includes KINGDOM Animalia reef-building corals, anemones, jellyfish, and hydroids, most PHYLUM Cnidaria of which are marine. Cnidarians have a radially symmetrical CLASSES 5 body shaped like a simple sac, with stinging tentacles around a SPECIES 10,886 single opening that serves as both mouth and anus. There are two body forms: the polyp form, typified by sea anemones, which is fixed to a solid surface and has an upward-facing mouth and tentacles; and the medusa, shown by adult jellyfish, which can swim and has a downward-facing mouth and tentacles. DOMAIN Eucarya
Anatomy Corals and anemones exist only as polyps, whereas other cnidarians can be either polyps or medusae at different stages of their life cycle. The body wall of both polyps and medusae consists of two types of tissue. On the outside is the epidermis, which acts like a skin to protect the animal. The inner tissue layer, lining the body cavity, is the gastrodermis, which carries out digestion and produces reproductive cells. Separating and connecting these two layers is a jellylike substance called the mesoglea. The tentacles have stinging cells called cnidocytes, which are unique to this phylum and give it its name. A simple nervous system responds to touch, chemicals, and temperature.
TENTACLE ARRANGEMENT
The number of tentacles on coral polyps varies from one group to another. The polyps of all soft corals (above) have eight tentacles, hence their alternative name of octocorals. Hexacorals (right) have tentacles arranged in multiples of six. POLYP tentacle
POLYP AND MEDUSA
Polyps are essentially cnidocyte a tube, closed at one epidermis end, that attaches to a mesoglea hard surface by a basal gastrodermis disk. They live singly or in colonies. Medusae budding are bell-shaped and juvenile usually have a thicker gut mesoglea; some also have a shelf of muscle basal for locomotion. disk
epidermis
mesoglea gastrodermis
gut mouth shelf of muscle (velum)
tentacle MEDUSA
epidermal cell coiled thread
BEFORE nematocyst DISCHARGE barbs
uncoiled hollow thread
AFTER DISCHARGE
STINGING CELLS
Each cnidocyte contains a bulblike structure, called a nematocyst, which houses a coiled, barbed thread. When triggered by touch or chemicals, the thread explodes outward and pierces the prey’s skin. The animal’s tentacles are then used to haul the victim in.
OCEAN LIFE
SCLERITES
Small slivers of calcium carbonate called sclerites are scattered through the tissues of soft corals and sea fans. Here, they are visible as white shards under the skin of this soft coral. BUILDING REEFS
Coral reefs are built by colonies of coral polyps that secrete a hard exoskeleton of calcium carbonate. As the tiny polyps divide and grow, the reef expands.
Many corals are harvested for sale as souvenirs, and the most valued species are being overcollected. Particularly desirable are certain soft corals, in which the calcareous supporting column is so strong and dense it can be carved and polished. They include the red or precious coral, Corallium rubrum (below). As yet, there are no international regulations controlling trade in this species, although some countries restrict its collection. Black corals (order Antipatharia) also have strong skeletons that can be carved.
CNIDARIANS
Locomotion
Reproduction
The most mobile cnidarians are free-living jellyfish and medusae, which mainly drift in water currents but also swim actively using a form of jet propulsion. Most colonial cnidarians, such as corals and sea fans, cannot move from place to place. However, they can expand and contract their polyps to feed or avoid danger, and some sea pens can withdraw the whole colony below the surface of the sediment in which they live. Unattached mushroom corals may move JELLYFISH SWIMMING slowly, or even right themselves if A jellyfish swims by using muscles overturned. Anemones can creep slowly to contract its bell, forcing water out over the seabed on their muscular basal and pushing it along. The muscles then disk, and a few species swim if attacked. relax and the bell opens again.
Members of the class Anthozoa, such as corals and anemones, reproduce by asexual budding. A genetically identical copy of the adult grows on the polyp’s body wall. This budding juvenile drops off or stays attached to form a colony. Anthozoans also reproduce sexually, producing eggs and sperm within the polyps. Fertilized eggs develop into hairy, oval larvae (planulae), which either swim free or are brooded internally and then released. Hydrozoans have a two-stage life cycle. Their polyps release tiny free-swimming medusae into the water which, when mature, shed eggs and sperm. The resulting fertilized eggs develop into planulae that settle in a new area to grow into polyps. In contrast, the medusa form of jellyfish is usually much larger than the fixed polyp form and the polyps bud asexually.
261
BUDDING JELLYFISH POLYPS
bell relaxed and flattened, ready to propel forward
bell begins to contract and force water out
Jellyfish polyps are minuscule, and their sole function is to reproduce asexually by budding off baby jellyfish.
bell fully contracted, with little water remaining inside
Zooxanthellae The massive skeletons secreted by reef-building corals require energy for their construction. Corals cannot catch enough plankton in clear tropical waters to provide this energy. Instead, they rely on tiny, symbiotic single-celled algae, called zooxanthellae, living in their cells. These algae manufacture organic matter by photosynthesis, and make more food than they need, so the excess is used by the coral. The algae benefit from a safe place to live and obtain “fertilizer” from the coral by using its nitrogenous waste products. If stressed by disease or high temperatures, corals expel their zooxanthellae, in a process called coral bleaching, and may die of starvation.
ZOOXANTHELLAE
In this image of coral polyps, the green patches are zooxanthellae living in the corals’ tissues. The zooxanthellae give color to the otherwise colorless polyps.
CNIDARIAN CLASSIFICATION Cnidarians are divided into five classes and a large number of orders and families. This phylum used to be called the Coelenterata, a name still used by some authorities. Many species remain undescribed. BOX JELLYFISH Order Cubozoa
ANTHOZOANS Order Anthozoa
41 species
7,095 species
These jellyfish have a cube-shaped bell with four flattened sides and a domed top. There are four tentacles or clusters of tentacles, one at each corner. Most are virulent stingers.
These colonial or solitary polyps are diverse in shape and have no medusa phase. Octocorals (soft corals, sea fans, and sea pens) have polyps with eight feathery tentacles; hexacorals (including hard corals and anemones) have polyps with multiples of six simple tentacles; ceriantipatharians have polyps with unbranched tentacles.
HYDROIDS Order Hydrozoa 3,516 species
These colonial cnidarians mostly resemble plant growths attached to the sea bed. A few have hard skeletons and resemble corals, and some colonies float at the surface like jellyfish. Most species have a free-living medusa stage.
48 species
Less than an inch high, stalked jellyfish have a bell-shaped body with eight clusters of short, knobbed tentacles around the rim of the bell. They attach to seaweeds by a stalk that extends from the “top” of the bell.
186 species
These mostly free-swimming medusae are shaped like a bell or saucer with a fringe of stinging tentacles. The edges of the mouth, located on the underside, are drawn out to form trailing mouth tentacles or oral arms.
OCEAN LIFE
STALKED JELLYFISH Order Staurozoa
JELLYFISH Order Scyphozoa
262 CLASS HYDROZOA
Blue Buttons Porpita porpita DIAMETER 3/ in 4
(2 cm)
DEPTH
Surface HABITAT
Surface waters DISTRIBUTION
Worldwide in warm waters
At first sight, blue buttons could be mistaken for a small jellyfish or even a piece of blue plastic. In fact, it is a hydrozoan colony that is modified for a free-floating existence. Swarms of these unusual creatures can be seen drifting on the water’s surface or can sometimes be found washed up on the shore. The animal is kept afloat by a buoyant circular disk. Around the edge hang protective stinging polyps modified as knobbed tentacles. In the center underneath hangs a large feeding polyp that acts as the mouth for the whole colony. In between this and the tentacles are circlets of reproductive polyps. Unlike the Portuguese man-of-war (see p.214) to which it is related, blue buttons do not have a powerful sting.
CLASS HYDROZOA
Stinging Hydroid Aglaophenia cupressina Up to 16 in
HEIGHT
(40 cm) DEPTH 10–100 ft (3–30 m) HABITAT
around among the corals on a reef. Individual polyps are arranged along one side of the smallest branches and extend their stinging tentacles to catch small planktonic animals. The sting is not usually dangerous to humans, but it results in an itchy rash that can irritate for up to a week.
CLASS SCYPHOZOA
CLASS SCYPHOZOA
Deep-sea Jellyfish
Moon Jellyfish
Periphylla periphylla
Aurelia aurita
8–14 in (20–35 cm)
HEIGHT
DIAMETER
Up to 12 in (30 cm)
3,000–23,000 ft (900–7,000 m)
DEPTH
Coral reefs
HABITAT
DEPTH
Near surface
Open water
HABITAT
Open water DISTRIBUTION
While most hydroids are harmless to touch, the stinging hydroid has a powerful sting. The colonies look like clumps of feathers or ferns dotted
This jellyfish belongs to a group called coronate jellyfish, which are shaped like a ballet tutu. The upper part of the bell is a tall, stiff cone and the lower part a wider, soft, crown-shaped base with a scalloped edge. The 12 thin tentacles are often held in an upright position. The insides of the deep-sea jellyfish are a deep red color, and this may hide the bioluminescent light given out by its ingested prey. The jellyfish itself can squirt out a bioluminescent secretion that may help to confuse any predators. Unlike many jellyfish, the deep-sea jellyfish does not develop from a fixed bottom-living stage.
CLASS SCYPHOZOA
Stalked Jellyfish Haliclystus auricula HEIGHT
Up to 2 in (5 cm) DEPTH
0–50 ft (0–15 m)
HABITAT
On seaweed or
seagrass DISTRIBUTION
north Pacific
OCEAN LIFE
Deep water worldwide, except
Tropical reefs in Indian Ocean and southwestern Pacific DISTRIBUTION
Coastal waters of north Atlantic and
DISTRIBUTION
Worldwide; polar distribution unknown
Arctic Ocean
Most jellyfish drift and swim freely in the water, but stalked jellyfish spend their lives attached by a stalk to vegetation. The body of the jellyfish is shaped like a tiny funnel made up of eight equally spaced arms joined together by a membrane. Each arm ends in a cluster of tentacles on the funnel rim, and in some species there is an extra anchor-shaped tentacle between these. This animal cannot swim, but it can move by bending over on its stalk and turning “headover-heels,” using the anchor tentacles to fix itself temporarily to the sea bed as it flips over and then reattaches its adhesive disk. Stalked jellyfish can be found attached to seaweed or seagrass in the intertidal zone and shallow water, where they feed by catching prey, such as small shrimp and fish fry, with their tentacles and passing it to the mouth inside the funnel. Undigested remains are expelled from the mouth.
tentacle held upright
scalloped edge of bell
The moon jellyfish is possibly the most widespread of all jellyfish and can be found in almost every part of the ocean except for very cold waters. It exists mainly in coastal waters and is sometimes cast ashore in large numbers because it is not a strong swimmer and lives near the surface. The body is shaped like a saucer with a fringe of fine, short tentacles, which it uses to catch plankton. It can also trap plankton in sticky mucus on its bell and slide this down into its mouth on the underside. The gonads show through the translucent bell as four opaque horseshoe shapes.
CNIDARIANS
263
gonads
stinging tentacle
frilly mouth lobe
CLASS SCYPHOZOA
Mauve Stinger Pelagia noctiluca DIAMETER
Up to 5 in (13 cm) DEPTH
Near surface HABITAT
Open water
This jellyfish produces bioluminescent light shows, which are often admired from passing boats, but it also has a reputation as a ferocious stinger. As well as having eight stinging tentacles, it is covered in tiny red spots that are bundles of stinging cells. The sting is painful but not dangerous. The mauve
OCEAN LIFE
DISTRIBUTION Northeastern Atlantic, Mediterranean, Indian Ocean, and western and central Pacific
stinger glows by producing luminous mucus from surface cells when it is knocked or disturbed by waves. Hanging down from the underside of the mushroom-shaped bell are four long, frilly mouth lobes, which are sometimes called oral arms. These also have stinging cells that paralyze and entangle small planktonic animals. Sticky mucus holds the prey, which is then passed up grooves in the arms and into the mouth. Unlike most jellyfish, the life cycle of the mauve stinger does not involve a fixed stage. Eggs and sperm are shed into the water, where the eggs are fertilized and develop into tiny, oval planula larvae covered in hairlike cilia. The planula larva changes directly into a tiny, lobed, saucer-shaped medusa called an ephyra, which gradually develops into an adult.
264
ANIMAL LIFE CLASS SCYPHOZOA
Upside-down Jellyfish Cassiopea xamachana DIAMETER
Up to 12 in
(30 cm) DEPTH
0–33 ft (0–10 m)
Coastal mangroves
HABITAT
DISTRIBUTION
Tropical waters of Gulf of Mexico and
Caribbean
Divers who find this jellyfish upsidedown on the seabed often think they have found a dying specimen. However, the upside-down jellyfish lives like this, floating with its bell pointing downward and its eight large,
branching mouth arms held upward. The mouth arms have elaborate fringes consisting of tiny bladders filled with minute single-celled algae called zooxanthellae. The algae need light to photosynthesize, and the jellyfish behaves as it does in order to ensure its passengers can thrive. Excess food manufactured by the algae is used by the jellyfish, but it can also catch planktonic animals with stinging cells on the mouth arms. Its bell pulsates to create water currents that bring food and oxygen. When it wants to move, the upside-down jellyfish turns the right way up with the bell uppermost. A very similar jellyfish, Cassiopeia andromeda, is found in the tropical Indian and Pacific Oceans and may actually be the same species.
CLASS CUBOZOA
Box Jellyfish Chironex fleckeri DIAMETER
Up to 10 in (25 cm) DEPTH
Near surface HABITAT
Open water Tropical waters of southwest Pacific and eastern Indian Ocean
DISTRIBUTION
A sting from the box jellyfish can kill a person in only a few minutes, and this small animal is considered one of the most venomous in the ocean. At each corner of its box-shaped, transparent body is a bunch of 15 tentacles. When it is hunting prey such as shrimp and small fish in shallow water, the tentacles extend up to 10 ft (3 m), and swimmers can be stung without ever seeing the jellyfish. In the middle of each flattened side is a collection of sense organs including some remarkably complex eyes. The exact range of this jellyfish in the Indo-Pacific region north of Australia is not known, but other smaller, less dangerous box jellyfish also occur in the Indian and Pacific oceans. Some sea turtles can eat the box jellyfish without being affected by its sting. HUMAN IMPACT
LETHAL VENOM The sting of a box jellyfish causes excruciating pain and skin damage and can leave permanent scars. In severe cases, death may occur from heart failure or drowning following loss of consciousness. A box jellyfish antivenin is available in Australia. In northern parts of the country, some beaches are closed to the public for periods between November and April when the jellyfish are most abundant.
CLASS ANTHOZOA
Organ Pipe Coral Tubipora musica DIAMETER
Up to 20 in (50 cm) DEPTH
15–65 ft (5–20 m) HABITAT
Tropical reefs Tropical reefs of Indian Ocean and western Pacific
DISTRIBUTION
CLASS ANTHOZOA
Mushroom Leather Coral
CLASS ANTHOZOA
Dead Man’s Fingers Alcyonium digitatum
Sarcophyton species
HEIGHT
Up to 8 in (20 cm)
DIAMETER
DEPTH
Up to 5 ft (1.5 m)
0–165 ft (0–50 m)
DEPTH
HABITAT
0–165 ft (0–50 m) HABITAT
Rocks and reefs Tropical waters of Red Sea, Indian Ocean, and western and central Pacific
Rocks and wrecks Temperate and cold waters of northeastern Atlantic
DISTRIBUTION
OCEAN LIFE
DISTRIBUTION
This distinctive soft coral has a conspicuous bare stalk topped by a wide, fleshy cap covered in polyps. When the colony is touched or is resting, the polyps are withdrawn into the fleshy body, and it looks and feels like leather. Within this genus there are many similar species.
This soft coral’s strange name comes from its appearance when thrown ashore by storms. It is shaped like a thick lump with stubby fingers, which can, with a little imagination, resemble a corpse’s hand. When alive, it grows attached to rocks in shallow water and often covers large areas, especially where strong currents bring plenty of planktonic
Although the Organ Pipe Coral has a hard skeleton, it is not a true stony coral. Instead, it belongs to a group of cnidarians called octocorals, which includes soft corals and sea fans. Its beautiful red skeleton is made up of parallel tubes joined by horizontal links, and bits of this animal’s skeleton are often found washed up on tropical shores. A single polyp extends from the end of each tube, and when the polyps expand their eight branched tentacles to feed, the skeleton cannot be seen. food. With the polyps extended, the colonies have a soft, furry look. Most dead man’s fingers colonies are white but some, like those shown below, are orange with white polyps. Over the fall and winter, the colony retracts its polyps and becomes dormant. In the spring, the outer skin is shed, along with any algae and other organisms that have settled on it.
CNIDARIANS
265
CLASS ANTHOZOA
Carnation Coral Dendronephthya species HEIGHT
Up to 12 in (30 cm) DEPTH
33–165 ft (10–50 m) HABITAT
Coral reefs DISTRIBUTION Tropical reefs of Red Sea, Indian Ocean, and western Pacific
Carnation corals are among the most colorful of all reef animals. They grow as branched and bushy colonies and often cover steep reef walls with pink, red, orange, yellow, and white patches. They prefer to live where there are fast currents. When the current is running, they expand to full size and the polyps, which are on the branch ends, extend out to feed. With little or no current, they often hang down as flaccid lumps. In some species, such as the one shown here, small slivers of colored calcium carbonate show through the body tissue. These are called sclerites and help to give the soft branches some strength. Individual species of Dendronephthya are difficult to identify visually and many species have not yet been described.
CLASS ANTHOZOA
Pulse Coral
CLASS ANTHOZOA
Common Sea Fan
Xenia species
Gorgonia ventalina HEIGHT
HEIGHT
Up to 2 in (5 cm)
Up to 6 ft (2 m)
DEPTH
DEPTH
15–165 ft (5–50 m)
15–65 ft (5–20 m)
HABITAT
HABITAT
Coral reefs
Coral reefs
DISTRIBUTION Tropical reefs of the Red Sea, Indian Ocean, and western Pacific
DISTRIBUTION
Caribbean Sea
Sea fans grow attached to the seabed and look like exotic plants. Unlike soft corals, they have a supporting skeleton that provides a framework and allows them to grow quite large. It is made mainly of a flexible, horny material
CLASS ANTHOZOA
White Sea Whip Junceella fragilis HEIGHT
Up to 6 ft (2 m) DEPTH
15–165 ft (5–50 m) HABITAT
Coral reefs DISTRIBUTION
Southwestern Pacific
Sea whips have a very similar structure to sea fans but grow up as a single tall stem. They have a very strong central supporting rod containing a lot of calcareous material as well as a flexible, horny material called gorgonin. The small polyps have eight tentacles and are placed all around the stem. White sea whips are often found in groups because they can reproduce asexually. As the whip enlarges, the fragile tip breaks off and drops onto the seabed, where it attaches and grows.
OCEAN LIFE
The most notable feature of this soft coral is the way the feathery tentacles of the polyps rapidly and continually open and close. A reef covered in Pulse Coral is alive with movement. The colonies have a stout trunk with a dome-shaped top covered with long polyps. Unlike the Mushroom Leather Coral (see opposite), Pulse Coral polyps cannot retract and disappear. The pulsating movements of the polyps may help to oxygenate the colony as well as bring food within range of their tentacles.
called gorgonin and consists of a rod that extends down the inside of all except the smallest branches. In the common sea fan, the branches are mostly in one plane and form a mesh that is aligned at right angles to the prevailing current. This increases the amount of planktonic food brought within reach of the polyps, which are arranged all around the branches. Fishing nets dragged over the reef can damage common sea fans and, as they grow quite slowly, they take a long time to recolonize. They are also collected, dried, and sold as souvenirs.
266
ANIMAL LIFE CLASS ANTHOZOA
Mediterranean Red Coral Corallium rubrum HEIGHT
Up to 20 in
(50 cm) 165–650 ft (50–200 m)
DEPTH
its name, it is not a true stony coral but instead is in the same group as sea fans (see p.265). Like them, its branches are covered in small polyps, each of which has eight branched tentacles. However, the supporting skeleton is made mainly from hard calcium carbonate colored a deep red or pink. This coral is now scarce in places that are easily accessible to collectors.
Shaded rocks and caves
HABITAT
Mediterranean and warm waters of eastern Atlantic
CLASS ANTHOZOA
CLASS ANTHOZOA
Slender Sea Pen
Giant Anemone
Virgularia mirabilis
Condylactis gigantea
HEIGHT
DIAMETER
Up to 24 in (60 cm)
(30 cm)
DEPTH
DEPTH
Up to 12 in
33–1,300 ft (10–400 m)
10–165 ft (3–50 m)
HABITAT
HABITAT
Sediment
rocks
Coral reefs and
DISTRIBUTION
Temperate waters of northeastern Atlantic and Mediterranean
DISTRIBUTION
Tropical waters of Caribbean Sea and western Atlantic
The muddy bottoms of sheltered sea lochs in Scotland and Norway are often carpeted in dense beds of slender sea pens. This species has a structure similar to the orange sea pen (see below, left) but has a much thinner central stalk and thin branches. Almost half the stalk is buried in the sediment and the colony can withdraw into the sediment if disturbed.
The long, purple-tipped tentacles of this large anemone bring a splash of color to Caribbean reefs. Its columnar body is usually tucked away between rocks or corals, leaving only the stinging tentacles exposed. Several small reef fish (mainly blennies) can live unharmed among the tentacles, where they gain protection from predators. The giant anemone can move slowly along on its basal disk if it wants to find a better position on the reef.
DISTRIBUTION
Often called precious coral, Mediterranean red coral has been collected and its skeleton made into jewelry for centuries. In spite of
CLASS ANTHOZOA
Orange Sea Pen Ptilosarcus gurneyi HEIGHT
Up to 20 in (50 cm) DEPTH
33–1,000 ft (10–300 m) HABITAT
Sediment DISTRIBUTION
Temperate waters of northeastern
Pacific
Unlike the majority of anthozoans, sea pens live in areas of sand and mud. They get their name from their resemblance
to an old-fashioned quill pen. The orange sea pen consists of a central stem with branches on either side. The basal part of the stem is bulbous and anchors the colony in the sediment. Single rows of polyps extend their eight tentacles into the water from each leaflike branch, giving the front of the sea pen a downy appearance. The colony faces toward the prevailing current to maximize the flow of plankton over the feeding polyps. When no current is flowing, the colony can retract down into the sediment. Although they tend to stay in one place, colonies can relocate and re-anchor themselves if necessary. Predators of sea pens include sea slugs and starfish.
CLASS ANTHOZOA
Beadlet Anemone Actinia equina DIAMETER
acrorhagi containing stinging cells
Up to 23/4 in (7 cm) DEPTH
OCEAN LIFE
0–65 ft (0–20 m) HABITAT
Hard surfaces Coastal waters of Mediterranean, northeastern and eastern Atlantic
DISTRIBUTION
Most anemones cannot survive out of water, but the beadlet anemone can do so provided it stays damp. At low tide, this anemone can be found on rocky shores with its tentacles
retracted, looking like a blob of red or green jelly. The top of the anemone’s body is ringed with blue beads called acrorhagi. These contain numerous stinging cells, which the anemone uses to repel any close neighbors. Leaning over, it will sting any anemone within reach, and the defeated anemone will move slowly out of the victor’s territory. The beadlet anemone broods its eggs and young inside the body and ejects them through its mouth.
CNIDARIANS CLASS ANTHOZOA
CLASS ANTHOZOA
Plumose Anemone
Cloak Anemone
Metridium senile
Adamsia palliata Up to 12 in
HEIGHT
DIAMETER
(30 cm) DEPTH
2 in (5 cm)
0–330 ft (0–100 m)
HABITAT
DEPTH
0–650 ft (0–200 m)
Any hard
surface
HABITAT
Hermit crab shells DISTRIBUTION
Temperate waters of north Atlantic and north Pacific
DISTRIBUTION
Temperate waters of northeastern Atlantic and Mediterranean
This tall anemone resembles an ornate piece of architecture. It has a long column, topped by a collarlike ring and a wavy disk with thousands of fine tentacles. The most common colors are white or orange, but it can also be brown, gray, red, or yellow. Fragments from the base of large anemones can grow into tiny new anemones. The plumose anemone is often found on pier pilings and wrecks projecting out into the current.
The cloak anemone lives with its wide base wrapped around the shell of a hermit crab and its tentacles trailing beneath the crab’s head. In this position, the tentacles are ideally placed to pick up food scraps. The enveloping column of the anemone is off-white with distinct pink spots. Neither partner thrives without the other, though young cloak anemones can be found on rocks and shells between the tidemarks waiting to find a host.
ARMORED VEHICLE The hermit crab Pagurus prideaux is always seen with its protective anemone cloak. It does not have to find a bigger shell as it grows because the cloak anemone secretes a horny extension. The anemone on the crab on the left has thrown out pink stinging threads, called acontia, to repel another hermit crab.
267
CLASS ANTHOZOA
Antarctic Anemone Urticinopsis antarctica SIZE
Not recorded DEPTH
15–740 ft (5–225 m) HABITAT
Rocky sea beds Southern Ocean around Antarctica and South Shetland Islands DISTRIBUTION
Like many other Antarctic marine animals, the Antarctic anemone grows to a large size, but rather slowly. It has long tentacles with powerful stinging cells and is capable of catching and eating starfish, sea urchins, and jellyfish much larger than itself. As there are often many anemones living close together, two or more may hold a large jellyfish. As in most anemones, stinging cells on the tentacles fire barbed threads into the prey to hold it and to paralyze or kill it.
CLASS ANTHOZOA
Jewel Anemone Corynactis viridis DIAMETER 1/ 2
in (1 cm)
DEPTH
0–260 ft (0–80 m) HABITAT
Steep rocky areas Temperate waters of northeastern Atlantic and Mediterranean DISTRIBUTION
OCEAN LIFE
Jewel anemones often cover large areas of underwater cliff faces, creating a spectacular display. Individuals can be almost any color, and they reproduce by splitting in half, making two new identical anemones. This results in dense patches of differentcolored anemones. Each anemone has a small saucer-shaped disk circled by stubby translucent tentacles. The tentacles have knobbed tips that are often a contrasting color to the tentacle shafts, disk, and column of the anemone. The color combination shown here is one of the most common. Jewel anemones are not true anemones but belong to a group of anthozoans called coralliomorphs. These closely resemble the polyps of hard corals but have no skeleton. Coralliomorphs are found in all oceans but are most common in the tropics.
268
ANIMAL LIFE CLASS ANTHOZOA
Table Coral Acropora hyacinthus DIAMETER
Up to 10 ft ( 3 m) DEPTH
0–33 ft (0–10 m) HABITAT
Coral reefs Tropical waters of Red Sea, Indian Ocean, and western and central Pacific DISTRIBUTION
The magnificent flat plates of table coral are ideally shaped to expose as much of their surface as possible to sunlight. Like most hard corals, the cells of table coral contain zooxanthellae that need light to photosynthesize and manufacture food for themselves and their host. Table coral is supported on a short, stout stem that is attached to the seabed by a spreading base. The horizontal plates have numerous branches that mostly project upward from the surface, so each plate,
or table, resembles a bed of nails. Each of these branches is lined by cup-shaped extensions of the skeleton called corallites, from which the polyps extend their tentacles in order to feed, mainly at night. The usual color of table coral is a dull brown or green, but it is brightened up by the numerous reef fish that shelter under and around its plates. However, the shade the plates cast means that few other corals can live underneath a table coral. There are many other similar species that are also called table coral, but Acropora hyacinthus is one of the most abundant and widespread.
CLASS ANTHOZOA
Hump Coral Porites lobata DIAMETER
Up to 20 ft (6 m) DEPTH
0–165 ft (0–50 m) HABITAT
Coral reefs Tropical waters of Red Sea, Persian Gulf, and Indian and Pacific oceans
DISTRIBUTION
PEOPLE
CHARLIE VERON Born in Sydney, Australia, in 1945, Charlie Veron has been dubbed the “King of Coral” for his lifelong work on coral reefs. He has formally named and described over 100 new coral species, including many from the genus Acropora. His three-volume book Corals of the World is a classic text. It can be difficult to tell that hump coral is a living coral colony because it looks just like a large, lumpy rock. Closer inspection will show that the coral grows as a series of large lobes formed into a dome. The living polyps are tiny, with tentacles that are only about 1/32 in (1 mm) long, and during the day, they are hidden in their shallow skeleton cups. At night, they extend their tentacles to feed and the colony takes on a softer appearance. Hump coral is an important reefbuilding species. CLASS ANTHOZOA
Daisy Coral Goniopora djiboutiensis DIAMETER
Up to 3 ft (1 m) DEPTH
15–100 ft (5–30 m) HABITAT
Turbid reef waters
OCEAN LIFE
DISTRIBUTION Tropical waters of Indian Ocean and western Pacific
In most corals it is difficult to see the tiny polyps, but the daisy coral has polyps that are a few inches long. The head of each polyp is dome-shaped with the mouth in the middle, surrounded by a ring of about 24 tentacles. These are arranged rather like the petals of a daisy. Unlike the majority of corals, the polyps extend to feed during the day, though they will quickly withdraw if touched. Daisy coral grows as a rounded lump, but the shape is difficult to see when the polyps are extended. While most corals need clear water to survive, this species often covers large areas where the water is made turbid by disturbed sediment.
269 CLASS ANTHOZOA
CLASS ANTHOZOA
Mushroom Coral
Giant Brain Coral
Fungia scruposa
Colpophyllia natans Up to 16 ft
DIAMETER
DIAMETER
Up to 1 in (2.5 cm)
(5 m)
DEPTH
DEPTH 3–180 ft (1–55 m)
0–80 ft (0–25 m)
HABITAT Seaward side of coral reefs
HABITAT
Sediment and rubble DISTRIBUTION Tropical waters of Red Sea, Indian Ocean, and western Pacific
DISTRIBUTION
Mushroom coral is unusual in that it lives as a single individual rather than a colony. Juveniles start life as a small stalked disk attached to dead coral or rock. By the time they reach about 11/2 in (4 cm) in diameter, they become detached. The animal feeds at night and the tentacles are withdrawn during the day, leaving the skeleton clearly visible, with the mouth at the center of the disk. The skeleton resembles the gills of a mushroom. Mushroom coral uses its tentacles to turn itself the right way up if it is overturned by waves.
This huge coral grows as giant domes or extensive thick crusts and can live for more than 100 years. The surface of the colony is a convoluted series of ridges and long valleys, as in other species of brain coral, and this is what gives it its name. The valleys and ridges are often differently colored and the ridges have a distinct groove running along the top. Typically, the valleys are green or brown and the ridges are brown. The polyp mouths are hidden in the valleys and the tentacles are only extended at night. In recent years, giant brain corals in the Tortugas Islands (south of the Florida Keys) have been attacked by a disease and some have died. Particularly large colonies are popular tourist attractions in islands such as Tobago. As well as attracting divers, the coral heads attract fish, and some gobies live permanently on the coral.
CLASS ANTHOZOA
Dendrophyllid Coral Dendrophyllia species HEIGHT
Up to 2 in (5 cm) DEPTH
10–165 ft (3–50 m) HABITAT
Steep rock faces Tropical waters in Indian Ocean and from western Pacific to Polynesia DISTRIBUTION
Tropical waters of Gulf of Mexico
and Caribbean
With their large, flamboyant polyps, corals of the genus Dendrophyllia look more like an anemone than a coral. Dendrophyllids belong to a group called cup corals. They grow as a low-branching colony with each tubular individual distinct, and they do not develop the massive skeleton of reef-building corals. They have no zooxanthellae and grow in shaded parts of reefs such as below overhangs and especially on steep cliff faces. During the day, the polyps are entirely rock or even a shipwreck. When the tentacles are expanded, these tiny corals look just like anemones, with each tapering, transparent tentacle ending in a small knob. Devonshire cup coral occurs in a variety of colors from white to orange.
CLASS ANTHOZOA
Devonshire Cup Coral Caryophyllia smithii
withdrawn and the coral looks like a dull reddish lump. As darkness falls, the polyps expand their orange tentacles to feed on plankton and make a spectacular display that often covers large areas. This genus of coral is very difficult to identify to species level and can also be confused with cup corals belonging to the genus Tubastrea. CLASS ANTHOZOA
Lophelia Coral Lophelia pertusa DIAMETER
At least 33 ft (10 m)
DIAMETER
DEPTH
11/4 in (3 cm)
165–10,000 ft (50–3,000 m)
DEPTH
0–330 ft (0–100 m) HABITAT
Rocks and
Deep-sea reefs
DISTRIBUTION
While most corals grow as colonies in tropical waters, the Devonshire cup coral is solitary and lives in temperate parts of the ocean. It grows with its cup-shaped skeleton attached to a
Lophelia reefs more than 8 miles (13 km) long and 100 ft (30 m) high have been recorded off the coast of Norway. Because it lives in deep, dark water, this cold-water coral has no zooxanthellae to help build its white, branching skeleton. It therefore grows
OCEAN LIFE
wrecks Northeastern Atlantic and Mediterranean
HABITAT
Atlantic, eastern Pacific, and western Indian Ocean; distribution not fully known DISTRIBUTION
very slowly, and such large reefs are many hundreds of years old. Each polyp has 16 tentacles, which it uses to capture prey such as zooplankton and even krill from the passing current. Stinging cells render the prey immobile and it is then transferred to the mouth. In recent years many of these slow growing reefs have been badly damaged by trawlers trying to catch deep-sea fish.
270
ANIMAL LIFE CLASS ANTHOZOA
White Zoanthid Parazoanthus anguicomus HEIGHT
1 in (2.5 cm)
65–1,300 ft (20–400 m) DEPTH
Shaded rocks, wrecks, and shells
HABITAT
DISTRIBUTION
Temperate waters of northeastern
Atlantic
Most zoanthids are found in tropical waters, but the white zoanthid is common in the north Atlantic. Its white polyps arise from an encrusting base and it has two circles of tentacles around the mouth. One circle is usually held upward while the other lies flat. As well as covering rocks and wrecks, this species also encrusts worm tubes and Lophelia reefs (see p.179).
CLASS ANTHOZOA
CLASS ANTHOZOA
Whip Coral
Bushy Black Coral
Cirrhipathes species
Plumapathes pennacea
LENGTH
HEIGHT
Up to 3 ft (1 m)
Up to 5 ft (1.5 m)
DEPTH
DEPTH
10–165 ft (3–50 m)
15–1,100 ft (5–330 m)
HABITAT
Coral reefs Tropical waters of eastern Indian Ocean and western Pacific DISTRIBUTION
Whip corals, or wire corals, belong to a group of anthozoans called antipatharians to which the black corals (see right) also belong. Whip coral grows as a single unbranched colony that can be either straight
branches. Made of a tough, horny material, the skeleton is valuable as it can be cut and polished to make jewelry, although this species is not widely used for this purpose.
HABITAT
or coiled as in the species belonging to the genus Cirripathes shown here (whip corals are difficult to identify and many species remain undescribed). The feeding polyps of whip corals and black corals can be seen easily because, unlike sea fans, they cannot retract their short, pointed tentacles. Gobies live among the tentacles, hanging onto the coral with suckerlike pelvic fins.
Coral reefs Tropical waters of Gulf of Mexico, Caribbean Sea, and western Atlantic
DISTRIBUTION
Bushy black coral grows as a plantlike colony with branches shaped like large bird feathers. There are many different species of black corals, and they get their name from the strong black skeleton that strengthens their
CLASS ANTHOZOA
Tube Anemone Cerianthus membranaceus HEIGHT
14 in (35 cm) DEPTH
33–330 ft (10–100 m) HABITAT
Muddy sand
OCEAN LIFE
DISTRIBUTION
Mediterranean and northeast Atlantic
The long, pale tentacles of the tube anemone make a spectacular display but at the slightest disturbance, the animal will disappear down its tube in an instant. Tube anemones look superficially like true anemones but are more closely related to black corals (see above). They live in tubes made of sediment-encrusted mucus that can be up to 3 ft (1 m) long even though the animals are only about a third of this length. The slippery lining of the tube allows the animal to retreat rapidly. As well as about 100 long, slender outer tentacles, the animal has an inner ring of very short tentacles surrounding the mouth. The outer tentacles may look dangerous, but the tube anemone feeds only on plankton and suspended organic debris.
271
Flatworms POSSESSING VERY
thin, sometimes transparent bodies, flatworms are among KINGDOM Animalia the simplest of animals. Marine species PHYLUM Platyhelminthes mostly belong to a colorful group called Xenacoelamorpha polyclad flatworms—leaf-shaped animals, CLASSES 8 common on coral reefs. Some are found in SPECIES 20,430 fresh water, and many are parasitic. In the oceans, parasitic flukes and tapeworms are common in fish, mammals, and birds. Most flatworms belong to the phylum Platyhelminthes but acoel flatworms (see below) are now separated into the phylum Xenacoelomorpha (or Acoelomorpha). DOMAIN Eucarya
FOOD SEARCH
This polyclad flatworm is searching for food using simple eyespots and chemical receptors on the margins of its head.
Anatomy
Reproduction
The flatworm has a simple, solid structure with no internal cavity. It is so thin that oxygen can diffuse in from the water, and there are no blood or circulatory systems. The head end contains sense organs; advanced species have primitive eyes. The gut opens to the outside at one end, the opening serving as both mouth and anus. In polyclad flatworms, this opening is in the middle underside BODY SECTION of the body. When feeding, they In flatworms, the space between the internal extend a muscular tube (pharynx) organs is filled with soft connective tissue out of the mouth to grasp their crisscrossed by muscles. food. Polyclad flatworms are dorso-ventral longitudinal covered in tiny hairs (or cilia) gut muscle muscle gut which, together with simple connective branch tissue muscles, help them to glide over almost any surface. The anatomy of tapeworms and flukes is adapted to suit their parasitic lifestyle.
Most flatworms are hermaphrodites, so every individual has both ovaries and testes. The reproductive system is complex for such a primitive animal and includes special chambers and tubules where the ripe eggs are fertilized. When two polyclad flatworms meet, they may briefly touch heads and bodies in a short ritual before mating. After mating, the eggs are released into the water, laid in sand, or stuck to rocks. In some flatworms, the eggs develop directly into juvenile worms but in others they develop initially into an eight-lobed planktonic larva. Called Müller’s larva, it swims for a few days and then settles onto COMPLEX APPARATUS Some flatworms undetake the seabed and flattens “penis fencing,” where out into a young each tries to stab the other and inject sperm. flatworm.
CLASS ACOELOMORPHA
CLASS RHABTITOPHORA
Acoel Flatworm
Candy Stripe Flatworm
Waminoa species
Prostheceraeus vittatus
Less than 1/4 in
LENGTH
(5 mm) DEPTH
LENGTH
Not recorded
Up to 2 in (5 cm)
On bubble coral (Pleurogyra sinuosa)
HABITAT
DISTRIBUTION
DEPTH
0–100 ft (0–30 m) HABITAT
Tropical Indian and Pacific oceans
These diminutive flatworms look like colored spots on the bubble coral on which they live. Their ultra-thin bodies glide over the coral surface as they graze, probably eating organic debris trapped by coral mucus. Acoel flatworms have no eyes and instead of a gut, they have a network of digestive cells. They are able to reproduce by fragmentation, each piece forming a new individual. The genus is difficult to identify to species level and the distribution is uncertain.
CLASS ACOELOMORPHA
Green Acoel Flatworm
Muddy rocks DISTRIBUTION Temperate waters of northeastern Atlantic and Mediterranean
Most brightly colored flatworms are found on tropical reefs, but the candy stripe flatworm is an exception and can be found as far north as Norway. Generally a cream color, it is marked with reddish brown, lengthwise stripes. The head end of its flattened, leaf-shaped body has a pair of distinct tentacles and groups of primitive eyes. As it crawls along, the flatworm pushes the edges of its body up into folds; it is also able to swim using sinuous movements of the body. Usually found in rocky areas, it has also been seen on sand.
Convoluta roscoffensis Up to 1/2 in
LENGTH
(1.5 cm) DEPTH
Intertidal
Sheltered sandy shores HABITAT
Northeastern Atlantic; probably more widespread than shown
DISTRIBUTION
flatworm on bubble coral
OCEAN LIFE
Although difficult to see individually, these flatworms show up when they collect together in puddles of water on sandy shores at low tide. Their bodies harbor tiny, single-celled algae (p. 248) that color them bright green. In warm, sunlit pools the algae can photosynthesize and pass some of the food they make to their host. These flatworms are very sensitive to vibrations and quickly disappear down into the sand if footsteps approach.
272
ANIMAL LIFE CLASS RHABDITOPHORA
Exquisite Lined Flatworm
CLASS RHABDITOPHORA
Divided Flatworm Pseudoceros dimidiatus
Pseudobiceros bedfordi
LENGTH
Up to 3 in (8 cm)
LENGTH
DEPTH
Up to 3 in (8 cm)
Not recorded
DEPTH
HABITAT
Not recorded HABITAT
Coral reefs DISTRIBUTION
DISTRIBUTION
Tropical waters of Indian and western
Pacific oceans
Tropical waters of Indian and western
Pacific oceans
Divers frequently come across this beautiful flatworm on coral reefs. Its striking pattern of pinkish transverse stripes and white dots against a black background make it easily recognizable. It is usually seen crawling over rocks in search of tunicates and crustaceans, but it is also a fairly good swimmer. Sometimes, the head end is reared up and a pair of flaplike tentacles can be seen.
CLASS RHABDITOPHORA
Thysanozoon Flatworm Thysanozoon nigropapillosum LENGTH
Up to 3 in (8 cm) DEPTH
3–100 ft (1–30 m) HABITAT
Coral reef slopes DISTRIBUTION
Coral reefs
Tropical waters of Indian and western
Pacific oceans
The highly convoluted edge of the very thin thysanozoon flatworm is prominently displayed with a white outline. The rest of the upper side
Most species of flatworms display a distinctive pattern of colors that is more or less the same in every individual. However, the color patterns of the divided flatworm vary greatly between individuals. The body is always black with an orange margin, but the width and arrangement of the yellow or white lateral stripes, zebralike bars, or narrow and wide longitudinal stripes is highly variable. These highly contrasting colors act as a warning to predators that divided flatworms are not good to eat. Like other flatworms, this species has numerous photoand chemosensitive cells in its head region, which help the worm to find food and avoid danger. of the body is black and covered in short papillae, or protuberances, each of which ends in a yellow tip. This gives the flatworm the appearance of being peppered with yellow spots. As is the case with most tropical reef flatworms, little is known of the biology of this species, but the thysanozoon flatworm has been found in association with colonial tunicates and is thought to feed on these and other colonial animals. It has been observed to swim well, rhythmically undulating its wide body. Much of what is known about this and other tropical reef flatworms has come from observations made by recreational divers and photographers. A similar species, Thyanozoon flavomaculatum, is found on Red Sea coral reefs.
CLASS RHABDITOPHORA
Imitating Flatworm Pseudoceros imitatus LENGTH
Up to 1 in (2 cm) DEPTH
Not recorded HABITAT
Coral reefs Waters around New Guinea and northern Australia, perhaps more extensive
DISTRIBUTION
The imitating flatworm has a creamy gray background color and black reticulations surrounding pale pustules.
imitation of the skin of the sea slug Phylidiella pustulosa, and the flatworm’s color pattern is also almost identical to that of the sea slug. The sea slug secretes a noxious chemical to deter potential predators, and it may be that the imitating flatworm gains protection by looking and feeling to the touch like the distasteful sea slug.
Unlike the majority of polyclad flatworms, which have a relatively smooth skin, the imitating flatworm has a bumpy surface covered in small pustules. This appearance is an SOURCE OF IMITATION
Phylidiella pustulosa is one of the most common and widespread sea slugs on Indo-Pacific reefs about 15–130 ft (5–40 m) deep.
CLASS RHABDITOPHORA
Giant Leaf Worm Kaburakia excelsa LENGTH
Up to 4 in (10 cm) DEPTH
be seen through the skin. It feeds in the same way as most polyclad flatworms, by everting its pharynx over its prey. Most intertidal flatworms in this region are only about 1 in (2 cm) long, making this species easy to identify. It is common on floating docks and in mussel beds.
Intertidal HABITAT
Under coastal rocks DISTRIBUTION
Temperate waters of northeastern
Pacific
This large, oval flatworm crawls around rocks, stones, and undergrowth on the Pacific shores of North America. Its color is reddish-brown to tan, marked with darker spots, and when it is fully spread out, the branches of its digestive system may
CLASS CESTODA
Broad Fish Tapeworm OCEAN LIFE
GOOD IMITATION
Diphyllobothrium latum LENGTH
Up to 33 ft (10 m) DEPTH
Dependent on host HABITAT
Parasitic DISTRIBUTION
host species
Probably worldwide, dependent on
Some flatworms, including tapeworms, have become highly modified and live as parasites. The broad fish tapeworm has a complex life history. It begins life as a fertilized egg that is eaten by tiny freshwater crustaceans, inside which the larvae hatch. Freshwater, estuarine, and migratory marine fish, (such as salmon) become infected by the larvae when they eat either the crustaceans or other infected fish. The adult tapeworm lives in fish-eating mammals and may infect humans who eat raw fish. Other tapeworm species live as adults in the guts of marine fish.
RIBBON WORMS
Ribbon Worms ALSO CALLED NEMERTEAN
DOMAIN Eucarya
worms, KINGDOM Animalia ribbon worms can reach great lengths PHYLUM Nemertea of at least 160 ft (50 m), although many are small and inconspicuous. CLASSES 2 While they are commonly slightly SPECIES 1,358 flattened, the longest are cylindrical and are often called bootlace worms. The majority of ribbon worms live in the sea under rocks, among undergrowth or in sediment, and some are parasitic. A few species live inside the shells of mollusks and crabs.
Anatomy
stylet
proboscis
273
nerve ganglion nerve
Nemertean worms have a long, excretory unsegmented body with strong muscles in organs the body wall that can shorten the worm to a proboscis sheath fraction of its full length. Unlike flatworms, ribbon worms have blood vessels and a complete gut with blood vessel mouth and anus. It is often difficult to distinguish ovary between the front and rear end of the worm, but most species have many simple eyes at the front. The most characteristic feature of these worms is a strong, tubular structure called a proboscis that lies in a sheath above gut the gut. It can be thrust out by hydrostatic pressure, either through the mouth or a separate SECTION opening, and is used to capture prey. BODY Ribbon worms have no body cavity In some species, the proboscis is or gills; a simple circulatory system armed with a sharp stylet. carries oxygen around the body.
Reproduction
WARNING PATTERN
Some ribbon worms have bright patterns that may serve as a warning to predators that they are toxic. Drab-colored species only emerge at night to hunt.
CLASS ANOPLA
Football Jersey Worm Tubulanus annulatus LENGTH
Up to 30 in
(75 cm) 0–130 ft (0–40 m)
DEPTH
Gravel, stones, and sediment
HABITAT
DISTRIBUTION Cold and temperate waters of north Atlantic and north Pacific
One of the most strikingly colored ribbon worms, the football jersey worm has a patterning of longitudinal white lines and regularly spaced white rings. It may be found lying in an untidy pile beneath stones on the lower shore and may also be seen scavenging when the tide is out. More usually it lives below the shore on almost any type of seabed, including mud, sand, and shell gravel. To camouflage itself, it secretes a mucous tube that becomes covered in surrounding sediment.
Most marine ribbon worms have separate sexes and their numerous, simple gonads produce either eggs or sperm. These are usually shed into the sea through pores along the sides of the body. Some species cocoon themselves together in a mucous net where the eggs are duly fertilized. In some types of ribbon worms, the eggs develop directly into juvenile worms, while others initially hatch into various types of larvae. The long, fragile bodies of ribbon worms tend SWIMMING LARVA to break easily but they have the useful Some ribbon worms ability to regenerate any lost parts. Some develop from a species even use regeneration as a method planktonic larva called a pilidium. It is able to of asexual reproduction, where the body swim by beating hairlike breaks up into several pieces and each structures, called cilia. piece develops a new head and tail.
CLASS ANOPLA
Bootlace Worm Lineus longissimus LENGTH
Up to 180 ft (55 m) DEPTH
Intertidal HABITAT
Sediments and stones DISTRIBUTION
Temperate waters of northeast
Atlantic
The bootlace worm makes up for its rather drab brown color by its incredible length. Only a fraction of an inch in diameter, it reaches at least 33 ft (10 m) in length, and is one of the longest animals known. On the shore it appears as a writhing mass of knots lying on muddy sediment beneath boulders. Like all anoplan worms, it has its mouth behind the brain. This worm is difficult to pick up, because it exudes large amounts of mucus when handled.
CLASS ENOPLA
Ribbon Worm Nipponnemertes pulcher LENGTH
Up to 31/2 in
(9 cm) DEPTH 0–1,900 ft (0–570 m)
Coarse sediments
HABITAT
DISTRIBUTION Temperate and cold waters of Arctic, Atlantic, Pacific and Southern oceans
beneath. This species has a distinctive, shield-shaped head with numerous eyes along its edges. The number of eyes increases with age. It is usually seen when dredged up by scientists from the coarse sediments in which it lives, but is sometimes found beneath stones on the lower shore. Its full distribution is unknown.
OCEAN LIFE
This worm belongs to a class of nemertean worms called enoplan ribbon worms, whose mouth is located in front of the brain. Nipponnemertes pulcher has a short, stout body with a width of up to 1/4 in (5 mm) that tapers to a pointed tail. The coloration varies from pink to orange or deep red and is paler
274
ANIMAL LIFE
Segmented Worms SEGMENTED WORMS
include two familiar, predominantly land-based KINGDOM Animalia and freshwater groups, the earthworms PHYLUM Annelida and the leeches. In the oceans, a third CLASSES 2 group, the bristleworms or polychaetes, are numerous and diverse. These SPECIES 15,000 include burrowing lugworms, free-living predatory ragworms, and tube-dwelling worms. All segmented worms share one main characteristic—the long, soft body is divided into a series of almost identical, linked segments. DOMAIN Eucarya
BRISTLEWORM
Fire worms have long, sharp bristles on each body section. These break off if the worm is attacked and can cause severe skin irritation.
Anatomy
Reproduction
Each body segment is called a metamere and, except for the head and tail tip, all are virtually indistinguishable from each other. In bristleworms, flattened lobes (parapods) project from the sides of each segment, and are reinforced by strong rods made of chitin. The worm uses parapods for locomotion, parapod and projecting bundles of bristles help it to grip. Internally, the segments are separated ventral by partitions and filled with fluid. nerve The gut, nerve cord, and large blood cord nerve vessels run all along the body.
In most polychaete worms, the sexes are separate and the eggs and sperm are shed into the water. Spawning is usually seasonal, especially at temperate latitudes. In many species, the fertilized egg develops into a larva (trochophore) that resembles epitoke a tiny spinning top. It floats and swims in the plankton, propelled by the beating of hairlike cilia around its middle. Eventually, the larva elongates and constricts into segments as it turns into an adult. Some species brood their eggs until the larvae are well developed. Many polychaete worms change shape as they become sexually mature, becoming little more than swimming READY TO BURST bags of eggs or sperm. The egg- or sperm-laden Known as epitokes, they epitoke of a palolo worm swarm, burst open to release separates from the front the eggs or sperm, then die. segments, and bursts open.
ganglion
epidermis
BODY SECTIONS
intestine
Most segments containexcretory organ their own organs, including excretory and(nephridium) reproductive organs, and branches from the main blood vessels and ventral nerve cord.
dorsal blood vessel segmental blood vessel
JAWS OF A PREDATOR
This bobbit worm seizes prey using a proboscis tipped with sharp mandibles, which it shoots out from the mouth. excretory organ (nephridium)
parapod ventral nerve cord
CLASS POLYCHAETA
CLASS POLYCHAETA
Lugworm
Sea Mouse
Arenicola marina
Aphrodita aculeata LENGTH
LENGTH
Up to 8 in (20 cm)
Up to 8 in (20 cm)
DEPTH
DEPTH
Shore and just below
Shallow to moderate
HABITAT
HABITAT
Muddy sand
Sand, muddy sand Temperate coastal waters of northeastern Atlantic and Mediterranean
Temperate shores of northeastern Atlantic, Mediterranean, and western Baltic
DISTRIBUTION
OCEAN LIFE
DISTRIBUTION
One of the most familiar sights on western European beaches is the neat, coiled casts of undigested sand deposited by lugworms. The worm itself is rarely seen, remaining hidden in its U-shaped tube beneath the surface of the sand. The entrance to the tube is marked by a shallow, saucer-shaped depression in the sand. The worm may be pink, red, brown, black, or green. The first six segments of its front section are thick with bristles, while the next thirteen segments have red, feathery gills. The rear third of the body is thin, with no gills or bristles. Lugworms feed by eating sand, extracting organic matter from it, and expelling the waste.These fleshy worms are a favorite food of many wading birds and are also used by fishermen as bait.They are most abundant at mid-shore level in sediments containing reasonable amounts of organic matter.
CLASS POLYCHAETA
Green Paddle Worm Eulalia viridis LENGTH
Up to 6 in (15 cm)
Shore and shallows
DEPTH
Rocky areas under stones, in crevices
HABITAT
DISTRIBUTION Temperate coastal waters of northeastern Atlantic
Although this beautiful green worm is usually found crawling over rocks, it can also swim well. The name paddle worm comes from the large, leafshaped appendages called parapodia that are attached to the side of each
body segment and aid in swimming. The head has two pairs of stout tentacles on each side, a single tentacle on top, and four short, forwardpointing tentacles at the front. These tentacles and two simple black eyes help the worm in its hunt for food. The green paddle worm is attracted to dead animals, especially mussels and barnacles, but will also hunt for live prey. However, unlike the king ragworm (opposite), it does not have jaws to tackle large prey. Instead, carrion and debris sticks to its proboscis and is wiped off inside the mouth. During spring, the green paddle worm lays gelatinous green egg masses about the size of a marble on the shore and in shallow water, attaching them to seaweeds and rocks.
The segmented structure of this pretty worm can be seen only if it is turned over, because its back is disguised by a thick felt of hairs that mask its segments. Running along each side of its body are numerous stiff, black bristles and a fringe of beautiful, iridescent hairs that glow green, blue, or yellow. The bristles can cause severe irritation if they puncture the skin. The sea mouse is so called because it looks like a bedraggled mouse when washed up dead on the seashore.
SEGMENTED WORMS CLASS POLYCHAETA
WORM REEFS
King Ragworm Alitta virens LENGTH
Up to 20 in (50 cm) DEPTH
Shore and shallows HABITAT
Muddy sand Temperate coastal waters of northeastern and northwestern Atlantic DISTRIBUTION
This large worm has strong jaws that are easily capable of delivering a painful bite to a human. The jaws are pushed out on an eversible proboscis and are used for pulling food into its mouth as well as for defending itself. The king ragworm lives in a mucuslined burrow in the sand, and waits for the tide to come in before coming out to feed. It swims well by bending its long body into a series of S-shaped curves. Fishermen collect it for bait.
crown of spines in three concentric rings
CLASS POLYCHAETA
Honeycomb Worm Sabellaria alveolata LENGTH
Up to 11/2 in
DEPTH Shore and shallows HABITAT Mixed rock and sand areas
Intertidal areas of northeastern Atlantic and Mediterranean
DISTRIBUTION
Magnificent Feather Duster Sabellastarte magnifica LENGTH
Up to 6 in (15 cm) DEPTH
3–65 ft (1–20 m) HABITAT
Coral reefs DISTRIBUTION Shallow waters of the western Atlantic and Caribbean
Honeycomb worms build their tubes by gluing together sand grains stirred up by waves. The glue is a mucus secreted by the worm, which uses a lobed lip around its mouth to fashion the tube. As new worms settle out from the plankton to build their own tubes, a reef develops and expands sideways and upward, provided there is a good supply of sand. These structures provide a home to many other species. LIVE REEF
fingerlike gills on each body segment
(4 cm)
CLASS POLYCHAETA
275
Live reefs will survive for many years provided new larvae settle and grow to replace wave-damaged areas and dead worms.
Although honeycomb worms are tiny, the sand tubes they build may cover many yards of rock in rounded hummocks up to 20 in (50 cm) thick. The worms build their tubes close together, and the tube openings give the colony a honeycomb appearance. This worm’s head is crowned by spines and it has numerous feathery feeding tentacles around the mouth, which it uses to trap plankton. The body ends in a thin, tubelike tail with no appendages.
The only part of this worm that is normally visible is a beautiful fan of feathery tentacles. The worm’s segmented body is hidden inside a soft, flexible tube that it builds tucked beneath rocks or in a coral crevice or buried in sand. The tentacles are in two whorls and are usually banded brown and white. They are normally extended into the water to filter out plankton, but at the slightest vibration or disturbance, such as the exhalation of a scuba diver, the worm instantly retracts the tentacles down into the safety of the tube.
CLASS POLYCHAETA
Pompeii Worm Alvinella pompejana LENGTH
Up to 4 in (10 cm)
6,500–10,000 ft (2,000–3,000 m)
DEPTH
Hydrothermal vent chimneys
HABITAT
DISTRIBUTION
CLASS POLYCHAETA
Christmas Tree Worm Spirobranchus giganteus LENGTH
Up to 11/4 in
(3 cm) DEPTH 0–100 ft (0–30 m) or more HABITAT
Living coral
heads DISTRIBUTION
Shallow reef waters throughout
the tropics
This extraordinary worm lives in thin tubes massed together on the sides of chimneys of deep-sea hydrothermal vents. The tubes are close to the chimneys’ openings, where water from deep inside Earth pours out at temperatures of up to 660˚F (350˚C). The temperature within the worm tubes reaches 176˚F (80˚C). At its head end, the Pompeii worm has a group of large gills and a mouth surrounded by tentacles. Each of the worm’s body segments has appendages on the side called parapodia. The posterior parapodia have many hairlike outgrowths that carry a mass of chemosynthetic bacteria. The bacteria manufacture food that the worm absorbs, and the worm also eats some of the bacteria.
OCEAN LIFE
Many large coral heads in tropical waters are decorated with Christmas tree worms, which occur in a huge variety of colors. The worm lives in a calcareous tube buried in the coral and extends neat, twin spirals of feeding tentacles above the coral surface. If disturbed, the worm pulls back into its tube in a fraction of a second. For added safety, the worm can also plug its tube with a small plate called an operculum.
Eastern Pacific
276
ANIMAL LIFE
Mollusks AMONG THE MOST SUCCESSFUL
of all marine animals, mollusks display great diversity and a remarkable range KINGDOM Animalia of body forms, allowing them to live almost everywhere PHYLUM Mollusca from the ocean depths to the splash zone. They include CLASSES 8 oysters, sea slugs, and octopuses. Most species have shells SPECIES 73,682 and are passive or slow-moving; some lack eyes. Others are intelligent, active hunters with complex nervous systems and large eyes. Filter-feeding mollusks, such as clams, are crucial to coastal ecosystems, as they provide food for other animals and improve water quality and clarity. Many mollusks are commercially important for food, pearls, and their shells. DOMAIN Eucarya
Anatomy Most mollusks have a head, a soft body mass, and a muscular foot. The foot is formed from the lower body surface and helps it to move. Mollusks have what is called a hydrostatic skeleton—their bodies are supported by internal fluid pressure rather than a hard skeleton. All mollusks have a mantle, a body layer that covers the upper body and may or may not secrete a shell. The shell of bivalves (clams and relatives) has two halves joined by a hinge; these can be held closed by powerful muscles while the tide is out, or if danger threatens. Mollusks other than bivalves have a rasping mouthpart, or radula, which is unique to mollusks. Cephalopods (octopuses, squid, and cuttlefish) also have beaklike jaws as well as tentacles, but most lack a shell, while most gastropods (slugs and snails) have a single shell. This is usually a spiral in snails, but can be cone-shaped in other forms, such as limpets. gill
The tropical giant clam is the largest bivalve and may measure more than 3 ft (1 m) across and weigh over 440 lb (220 kg).
GASTROPOD ANATOMY
spiral shell
mantle cavity
sensory tentacle
digestive system
eye SPIRAL SNAIL SHELL
muscular foot
radula hinge ligament
BIVALVE ANATOMY
Bivalves are housed within a shell of two halves (right) from which the siphons and muscular foot can be extended. The shell is opened and closed by the adductor muscles, labeled in the body plan (far right).
REEF-DWELLING GOLIATH
The body plan (far left) of gastropods (slugs and snails) features a head, large foot, and usually a spiral shell (left). In shelled forms, all the soft body parts can be withdrawn into the shell for protection, or to conserve moisture while uncovered by the outgoing tide.
shell mantle cavity
digestive system
siphon
muscular foot BIVALVE SHELL gill
adductor muscle
jaws feeding arm
OCEAN LIFE
radula digestive system
eye arm
internal shell
siphon gill
mantle cavity
CEPHALOPOD ANATOMY
Cephalopods have large eyes, in front of which there are a number of tentacles. The siphon functions in respiration and in rapid movement. Some forms have a flattened internal shell.
MOLLUSKS
Sense Organs
277
HUMAN IMPACT
Touch, smell, taste, and vision are well developed in many mollusks. The nervous system has several paired bundles of nervous tissue (ganglia), some of which operate the foot, and interpret sensory information such as light intensity. Photoreceptors range from the simple eyes (ocelli) seen along the edges of the mantle or on bivalve siphons, to the sophisticated image-forming eyes of cephalopods. Cephalopods are also capable of rapidly changing their color. PIGMENTED SKIN CELLS HELP CUTTLEFISH TO CHANGE COLOR
GRAFTING OYSTERS Pearls form in oysters when a grain of sand or other irritant lodges in their shells. The oyster coats the grain with a substance called nacre, forming a pearl. Today many pearls are cultured artificially: the shell is opened just enough to introduce an irritant into the mantle cavity. SEEDING AN OYSTER
The best-shaped artificial pearls are produced by “seeding” oysters with a tiny pearl bead and a piece of mantle tissue from another mollusk .
1
The giant cuttlefish’s color change is due to skin cells called chromatophores. It is pale when pigment is confined to a small area of each cell.
When the cuttlefish passes over a darker background, it disperses the colored pigments throughout each of its chromatophores, and the animal darkens.
2
MOLLUSCAN BEAUTY
Displaying fabulous warning colors, this nudibranch is a shell-less example of the many thousands of marine species of gastropods (slugs and snails).
Movement Mollusks move in many different ways. Most gastropods glide across surfaces using their mucus-lubricated foot. Exceptions include the sea butterfly, which has a modified foot with finlike extensions for swimming. Some bivalves, such as scallops, also swim, producing jerky movements by clapping the two halves of their shell together. Other bivalves burrow by probing with their foot and then pulling themselves downward by muscular action. Cephalopods are efficient swimmers; some have fins on the sides of their bodies that let them hover in the water, and they can accelerate rapidly by squirting water out through their siphons.
siphon
REDUCING DRAG
Swimming backward reduces drag from the tentacles. The siphon, used for jet propulsion, is clearly visible in this Humboldt squid.
AIDED BY MUCUS
Muscular contractions ripple through the fleshy foot of this marine snail. It secretes a lubricating mucus that helps it to move on rough surfaces.
Respiration Most mollusks obtain oxygen from water using gills, called ctenidia, which are situated in the mantle cavity. These are delicate structures with an extensive capillary network and a large surface area for gaseous exchange. In species that are always submerged, water can continually be drawn in and over the gills. Those living in the intertidal zone are exposed to the air for short periods and must keep their gills moist. At low tide, bivalves clamp shut and some gastropods close their shell with a “door” (called an operculum) to retain moisture. Pulmonate snails have a simple lung formed from the mantle cavity instead of ctenidia and are mostly terrestrial but others live on the seashore and can absorb oxygen through their skin when immersed.The respiratory pigment in most molluscan blood is a copper compound called hemocyanin. It is not as efficient at taking up oxygen as external gills hemoglobin and gives mollusks’ (ctenidia) blood a blue color.
Nudibranchs (sea slugs) have feathery external gills toward the rear of their bodies. The warning coloration of this species includes the bright orange gills.
OCEAN LIFE
COLOR CODING
278
ANIMAL LFE
Feeding
OCEAN LIFE
The ways in which molluscs feed are almost as varied as their anatomy. Sedentary molluscs, such as many bivalves including clams and oysters, create water currents through tubular outgrowths of their mantle (siphons). They filter food from the moving water with their mucus-covered gills. Suitably sized particles are then selected and passed to the mouth by bristly flaps called palps. Sea slugs, chitons, and many sea snails graze algae from hard surfaces using their rasplike radula. Radulae have tooth-like structures called denticles, many of which are reinforced with an iron deposit for durability. Larger molluscs feed on crustaceans, worms, fish, and other molluscs, which they locate either by scent or, in the case of some cephalopods such as octopuses, by sight. Cephalopods use their suckered arms to capture prey and their parrot-like beak to crush and dismember it. Some squid even appear to hunt in packs and swim in formation over reefs looking for prey.
SPECIES-SPECIFIC DENTICLES
The denticles on a mollusc’s radula are often species-specific. This electron micrograph shows the distinctive radula of the gastropod Sinezona rimuloides.
FEEDING TRAIL
Limpets continually graze the same area as the algae and bacterial film on which they feed regrow rapidly. The abrasive radula of the limpet scrapes a trail on the rock surface, as shown above.
MOLLUSCS LIMPET CHAIN
Reproduction
DEVELOPING EMBRYOS
In 4 months, Australian Giant Cuttlefish eggs develop into mini-replicas of the adults.
279
Slipper Limpets change from male to female as they grow. This chain of four such limpets has a female at the bottom and smaller males above her.
In many molluscs, reproduction simply involves releasing sperm or eggs (gametes) into the water. Fertilization is external and there is no parental care. Individuals may be of separate sexes or hermaphrodites (having both male and female reproductive organs). Hermaphrodites may function as male or female at different times or, as in nudibranchs, produce both eggs and sperm, although eggs can be fertilized only by cross-fertilization. Some species, such as slipper limpets, change sex with age, while oysters can change sex several times in a breeding season. Among cephalopods, males court females, fertilization is internal, and in some species, the eggs are protected by the females until they hatch.
Lifecycles
HUMAN IMPACT
OYSTER DEMAND Oysters have long been harvested as a food source. Their high market value and increasing demand have led to overexploitation of wild stocks. In the North Sea, the European flat oyster has vanished from much of its former range, and today most oysters are commercially farmed. SLOW RECOVERY PERIOD
Relatively long-lived and reproducing only sporadically, the European flat oyster (right) takes a long time to recover from overexploitation.
READY AND WAITING FOR PREY
This cuttlefish hovers with its arms outstretched. When prey comes within reach, the two feeding arms, currently contracted and set above the two lower arms, will shoot forward to grab the prey.
Most molluscs produce eggs that either float or are deposited in clusters, anchored to the substrate. Most forms have eggs that hatch into shell-less larvae, which live in the plankton. The larvae are called ciliated trochophores due to their bands of hair-like cilia, used in PLANKTONIC LARVA swimming. In gastropods, bivalves, The visible bands of this veliger larva of the and scaphopods, the trochophore Common Limpet beat with tiny hair-like cilia, which are used in locomotion and feeding. larvae change into veliger larvae, which have larger ciliated bands, and sometimes adult features such as a mantle or a rudimentary shell or both. As they approach maturity, the larvae float down from the surface and, on reaching the sea bed, change into adults. Only those that land in a suitable environment survive to reach sexual maturity. Cephalopod eggs hatch into active predators. Some resemble mini-adults; others live in the plankton and initially look and behave differently from the adults. SECURING EGG CLUSTERS each finger-shaped egg capsule holds up to seven eggs
This female Bigfin Reef Squid produces up to 400 egg capsules containing about 2,500 eggs. Here, she is securing egg capsules to a solid substrate.
MOLLUSC CLASSIFICATION The phylum Mollusca is the second largest animal phylum, comprising more than 73,000 species, and their diverse form has led to the identification of eight different classes. The majority of species live in marine habitats, but freshwater and terrestrial species are also numerous. MONOPLACOPHORANS Class Monoplacophora
131 species
About 30 species
These are marine, shell-less, worm-like organisms of deep-water sediments. Their horny outer layer is covered with spines.
These deep-sea molluscs lack eyes but have a radula and a cone-like shell. They are more abundant as fossils than as living species.
SOLENOGASTERS Class Solenogaster
TUSK SHELLS Class Scaphopoda
273 species
571 species
Another marine class of shell-less, wormlike organisms, solenogasters live in or on the ocean floor. Some lack a radula.
These animals have a tubular, tapering shell, open at both ends. The head and foot project from the wider end and dig in soft sediments.
CEPHALOPODS Class Cephalopoda
9,209 species
816 species
Bivalves, or clams and their relatives, have a hinged shell of two halves, but no radula. Most are sedentary and marine. Siphons create a water current through the shell, aiding feeding and respiration. Sexes are usually separate.
Squid, octopuses, and cuttlefish are all cephalopods – fast-moving and intelligent, with a complex nervous system and large eyes. The shell is internal or absent, the head surrounded by arms, with or without suckers. The central mouth has a parrot-like beak and a radula. The sexes are separate.
GASTROPODS Class Gastropoda 61,682 species
Familiar as slugs and snails, these molluscs are marine, freshwater, and terrestrial. They have a spiral shell and a large, muscular foot. The body is twisted 180º so the mantle cavity lies over the head. Many species can retract into their shell; hermaphrodite species are common.
CHITONS Class Polyplacophora 970 species
Chitons have a repeating structure with a series of plates (usually 8) on their backs enclosed by an extension of the mantle. The underside is dominated by the foot.
OCEAN LIFE
CAUDOFOVEATES Class Caudofoveata
BIVALVES Class Bivalvia
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ANIMAL LIFE CLASS BIVALVIA
Common Mussel Mytilus edulis LENGTH
10–15cm (4–6in) HABITAT
Intertidal zones, coasts, estuaries DISTRIBUTION North and southeastern Atlantic, northeastern and southwestern Pacific
Also called the Blue Mussel, this edible, black-shelled bivalve attaches itself in large numbers to various substrates using tough fibres called byssus threads. These fibres are extremely strong and prevent the mussels from being washed away. The fibres increase in strength in autumn perhaps to cope with storms. When the mussel opens its shell, water is drawn in over the gills, or ctenidia, which absorb oxygen into the tissues and also filter food particles out of the water. Common Mussels are very efficient filter feeders – they process about 45–70 litres (10–15 gallons) of water per day and consume almost everything they trap. The sexes are separate and so grouping together in “beds” helps to ensure that their eggs are fertilized. After hatching, the planktonic larvae are dispersed by the ocean currents. After some months the larvae settle, attach, and metamorphose, but can resorb their byssus and move to a better spot.
CLASS BIVALVIA
Black-lip Pearl Oyster Pinctada margaritifera LENGTH
Up to 30cm (12in) diameter HABITAT
Hard substrata of interand subtidal zones; reefs CLASS BIVALVIA
Great Scallop
DISTRIBUTION Gulf of Mexico, western and eastern Indian Ocean, western Pacific
Black-lip Pearl Oysters begin life as a male before changing into a female two or three years later. Females produce millions of eggs, which are fertilized randomly and externally by the males’ sperm, before hatching into free-swimming larvae. The mobile larvae pass through various larval stages for about a month before eventually settling on the sea floor, after metamorphosing into the sessile (immobile) adult form. This species is famous and much sought-after because it occasionally produces prized black pearls.
Pecten maximus WIDTH
Up to 17cm (71/2 in) HABITAT
Sandy sea beds, at 5–150m (16–500ft), commonly 10m (33ft) DISTRIBUTION
CLASS BIVALVIA
Atlantic Thorny Oyster Spondylus americanus
Northeastern Atlantic
OCEAN LIFE
LENGTH
Also known as the King Scallop, the Great Scallop is usually found partly buried in sand. It is one of the few bivalves capable of rapid movement through water, which it achieves using a form of jet propulsion. It claps the two halves of its shell together, which pushes water out of the mantle cavity close to the hinge. It moves forwards with its shell gape first, producing j erky movements as it takes successive “claps” of water. These movements are a useful strategy to escape from predators. These edible bivalves are now farmed to meet growing demand.
Up to 11cm (41/2 in) HABITAT
Rocks to a depth of 140m (460ft) DISTRIBUTION Southeast coast of USA, Bahamas, Gulf of Mexico, Caribbean
The Atlantic Thorny Oyster’s spiny shell protects it from predators.The oyster pictured here is covered with an encrusting red sponge, which provides camouflage. This species is unusual in having a ball-and-socket type hinge joining the two halves of its shell,
rather than the more common toothed hinge seen in many other bivalves. The Atlantic Thorny Oyster cements itself directly to rocks rather than using byssal threads.
CLASS BIVALVIA
Shipworm Teredo navalis LENGTH
60cm (24in) HABITAT
Wood burrows in high-salinity seas and estuaries Coastal waters off North, Central, and South America, and Europe
DISTRIBUTION
Despite its worm-like appearance, the shipworm is a type of clam that has become elongated as an adaptation to its burrowing lifestyle. Its bivalve shell, situated at the anterior end, is very small and ridged. The Shipworm uses it with a rocking motion to bore into wooden objects. Outside the shell its body is unprotected, except for a calcareous tube it secretes to line the burrow. These worms damage wooden structures, such as piers, irreparably and in the past caused many ships to sink.The burrow entrance is only about the size of a pinhead, but the burrow itself may be over 1cm (1/2 in) wide, so the extent of an infestation is often underestimated until it is too late. Shipworms change from male to female during their lifetime, and the female form produces many eggs, from which free-swimming larvae hatch. When they mature and settle on a suitable piece of wood, the larvae quickly metamorphose into the adult form and start burrowing.
MOLLUSCS CLASS BIVALVIA
Common Piddock Pholas dactylus LENGTH
Up to 15cm (6in) across HABITAT
Lower shore to shallow sublittoral DISTRIBUTION South and east coasts of UK, Severn estuary in UK, west coast of France, Mediterranean
anterior beak of elliptical shell
This mollusc has a pronounced “beak” covered in tooth-like projections at the front end of its shell. It uses this feature for boring holes into relatively soft substrates, such as mud, chalk, peat, and shale. Like the Shipworm (opposite), this piddock relies on its burrows for protection from predation, because the shell does not encase all of its body – its two fused siphons (tubes for eating, breathing, and excretion) trail out behind it. The shell is fragile, elliptical, and covered in a pattern of concentric ridges and radiating lines. If disturbed, the Common Piddock has an unusual defence strategy: it squirts a luminous blue secretion from its outgoing, or exhalant, siphon. Such bioluminescence is very rare in molluscs, seen in only a few species.
fused siphons
CLASS BIVALVIA
Giant Clam Tridacna gigas LENGTH
Up to 1.5m (5ft) HABITAT
Reefs, reef flats and shallow lagoons to 20m (65ft) DISTRIBUTION Tropical Indo-Pacific from south China seas to northern coasts of Australia, and Nicobar Islands in the west to Fiji in the east
The largest and heaviest of all molluscs is the Giant Clam. Like other bivalves, it feeds by filtering small food particles
from the water using its ingoing, or inhalant, siphon, which is fringed with small tentacles. However, it differs in obtaining most of its nourishment from zooxanthellae (unicellular algae that live within its tissues) – a type of relationship also associated with coral polyps. The algae have a constant and safe environment in which to live; in return they provide the clam with essential nutrients, the carbon-based products of photosynthesis. In fact, so dependent is the Giant Clam on these algae that it will die without them. The adult is sessile (immmobile) and its inhalant and exhalant (outgoing) siphons are the only openings in its mantle. Although the scalloped edges
CLASS BIVALVIA
281
CLASS BIVALVIA
Common Edible Cockle
Atlantic Jackknife Clam
Cerastoderma edule
Ensis directus
LENGTH
LENGTH
Up to 5cm (2in)
16cm (6in)
HABITAT
HABITAT
Middle and lower shore, 5cm (2in) below surface of sand or mud
Sandy and muddy shores and shallows
DISTRIBUTION Barents Sea, eastern north Atlantic from Norway to Senegal, West Africa
DISTRIBUTION Atlantic coast of North America, introduced to North Sea
This edible bivalve has a robust, ribbed shell and burrows in dense populations just below the surface of sand or mud, filtering organic matter such as plankton from the water. Cockles are an important commercial species but also a vital food source for wading birds such as oystercatchers.
Atlantic Jackknife Clams live in deep, vertical burrows on muddy and soft, sandy shores. Native to the northeast coast of North America, the freeswimming larval stage is thought to have been introduced to the North Sea in 1978 when a ship emptied its ballast tanks outside the port of Hamburg. This clam has spread along the continental coast. In places, it affects local polychaete worm populations, but it is not considered a pest.
of their shell halves are mirror images of one another, larger individuals may be unable to close their shells fully, so their brightly coloured mantle and siphons remain constantly exposed. Many Giant Clams appear irridescent due to an almost continuous covering of purple and blue spots on their mantles, while others look more green or gold, but all have a number of clear spots, or “windows”, that allow sunlight to filter into the mantle cavity. Fertilization is external and the eggs hatch into free-swimming larvae before settling onto the sea bed.The exhalant siphon expels water and at spawning time provides an exit point for the eggs or sperm.
SPAWNING Reproduction in Giant Clams is triggered by chemical signals that synchronize the release of sperm and eggs into the water. Giant Clams start life as males and later become hermaphroditic, but during any one spawning event, they release either sperm or eggs in order to avoid self-fertilization. A large clam can release as many as 50 million eggs in 20 minutes.
OCEAN LIFE
GIANT CLAM
The giant clam grows to its huge size with the help of photosynthetic microorganisms (zooxanthellae) living in its colorful mantle tissue, which share the sugars they produce with the clam. Collected for its meat and shell, it is now rare throughout its range and international trade is restricted.
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ANIMAL LIFE orange foot with greenish tint
CLASS GASTROPODA
Common Limpet Patella vulgata
CLASS GASTROPODA
Top Shell Tectus niloticus
DIAMETER
LENGTH
21/2 in (6 cm)
6 in (16 cm) HABITAT
HABITAT
Rocks on high shore to sublittoral zone
Intertidal and shallow subtidal areas, reef flats to 23 ft (7 m)
conical shell
MUSCULAR FOOT DISTRIBUTION Northeastern Atlantic from Arctic Circle to Portugal
Abundant on rocks from the high to the low water mark, the common limpet is superbly adapted to shore life. A conical shell protects it from predators and the elements. Limpets living at the low water mark are buffeted by the waves and so require smaller, flatter shells than those living at the high water mark, where wider,
The common limpet’s muscular foot, seen here from below, holds it firmly to its rock, regardless of the strength of the waves.
taller shells allow for better water retention during periods of exposure. Limpets travel slowly during low tide, covering up to 24 in (60 cm) using contractions of their single foot. They graze on algae from rocks using a radula (a rasplike structure), which has teeth reinforced with iron minerals.
Eastern Indian Ocean, western and southern Pacific
DISTRIBUTION
Easily distinguished from most other gastropods by the conical shape of its spiral shell, the top shell moves slowly over reef flats and coral rubble, feeding on algae. Demand for its flesh and pretty shell has led to declining numbers, especially in the Philippines, due to unregulated harvesting. It has, however, been successfully introduced elsewhere in the Indo-Pacific, such as French Polynesia and the Cook Islands, from where some original sites are being restocked.
CLASS GASTROPODA
Red Abalone Haliotis rufescens LENGTH
6–8 in (15–20 cm) HABITAT
Rocks from low tide mark to 100 ft (30 m) East Pacific coasts from southern Oregon, US to Baja California, Mexico
DISTRIBUTION
The largest of the abalone species, the red abalone is so called because of the brick-red color of its thick, roughly oval shell. There is an arc of
CLASS GASTROPODA
RETURNING HOME
Limpets gradually grind a “scar” into their anchor spot on the rock, to aid their grip and help retain water. A mucus trail leads them back to the spot.
Venus Comb Murex pecten LENGTH
Up to 3 in (8 cm) HABITAT
Tropical warm waters to 650 ft (200 m) CLASS GASTROPODA
CLASS GASTROPODA
Zebra Nerite
Dog Whelk
Puperita pupa
Nucella lapillus
DISTRIBUTION
LENGTH
LENGTH
Up to 1/2 in (1 cm)
Up to 21/2 in (6 cm)
HABITAT
HABITAT
Rocky tide pools
Middle and lower rocky shores
Caribbean, Bahamas, Florida
DISTRIBUTION
Northwestern and northeastern
OCEAN LIFE
Atlantic
The small, rounded, smooth, blackand-white striped shell of the zebra nerite is typical of the species, but in examples from Florida the shell is sometimes more mottled or speckled with black. These gastropods are most active during the day, when they feed on microorganisms such as diatoms and cyanobacteria, but if they become too hot or they are exposed at low tide, they cluster together, withdraw into their shells, and become inactive. This may be a mechanism for preventing excessive water loss. Unusually for gastropods, there are separate males and females of zebra nerites and fertilization of the eggs occurs internally. The males use their penis to deposit sperm into a special storage organ inside the female. Later, she lays a series of small white eggs that hatch into planktonic larvae.
One of the most common rocky shore gastropods, the dog whelk has a thick, heavy, sharply pointed spiral shell. The shell’s exact shape depends on its exposure to wave action, and its color depends on diet. Dog whelks are voracious predators, feeding mainly on barnacles and mussels. Once the prey has been located, the whelk uses its radula to bore a hole in the shell of its prey before sucking out the flesh.
DISTRIBUTION
Eastern Indian Ocean and western
Pacific
The tropical carnivorous snail known as the Venus comb has a unique and spectacular shell. There are rows of long, thin spines along its longitudinal ridges, which continue onto the narrow, rodlike, and very elongated siphon canal. The exact function of these spines is unknown, but they are thought to be either for protection or to prevent the snail from sinking into the soft substrate on which it lives. Its body is tall and columnar so that it can lift its cumbersome shell above the sediment to move in search of food.
three to five clearly visible holes in the shell, through which water flows for respiration.These are filled and replaced with new holes as the abalone increases in size. Sea otters are one of the red abalone’s main predators, along with human divers.
HIDING FROM VIEW There are times when the Venus comb buries itself just below the surface of the sea floor, displacing the sand with movements of its muscular foot. However, it leaves the opening of its tubular inhalant siphon above the sand’s surface so that it can draw water into its mantle cavity to obtain oxygen and to “taste” the water for the presence of prey.
HALF BURIED
The spines of this Venus comb can be seen sticking out of the sand. The siphon is visible to the right of the picture.
MOLLUSKS
285
CLASS GASTROPODA
Tiger Cowrie Cypraea tigris LENGTH
Up to 6 in (15 cm) HABITAT
Low tide to 100 ft (30 m) on coral reefs and flats
DISTRIBUTION
Indian Ocean, western Pacific
One of the largest cowrie species, the tiger cowrie has a shiny, smooth, domed shell with a long, narrow aperture, and is variously mottled in black, brown, cream, and orange. The cowrie’s mantle (its body’s outer, enclosing layer) can extend to cover parts of the exterior of the shell. These extensions have numerous projections, or papillae, whose exact function is unknown, but which may increase the surface area for oxygen absorption or provide camouflage of some sort. Tiger cowries are nocturnal creatures, hiding in crevices among the coral during the day and emerging at night to graze on algae. The sexes are separate and fertilization occurs internally. Females exhibit some parental care in that they protect their egg capsules by covering them with their muscular foot until they hatch into larvae, which then enter the plankton to mature.
CLASS GASTROPODA
Giant Triton Charonia tritonis LENGTH
Up to 16 in (40 cm) HABITAT
Coral reefs, mostly in subtidal zones
DISTRIBUTION
Indian Ocean, western and central
Pacific
CLASS GASTROPODA
Common Periwinkle Littorina littorea CLASS GASTROPODA
Flamingo Tongue Cyphoma gibbosum LENGTH
1–11/2 in (3–4 cm) HABITAT
DISTRIBUTION Western Atlantic, from North Carolina to Brazil; Gulf of Mexico, Caribbean Sea
The off-white shell of the flamingo tongue cowrie is usually almost completely hidden by the two fleshy, leopard-spotted extensions of its
LENGTH
Up to 1 in (3 cm) HABITAT
Upper shore to sublittoral rocky shores, mud flats, estuaries DISTRIBUTION Coastal waters of northwest Europe; introduced to North America
The common periwinkle has a black to dark gray, sharply conical shell and slightly flattened tentacles, which in juveniles also have conspicuous black banding. The sexes are separate and fertilization occurs internally. Females release egg capsules, containing two or three eggs,
directly into the water during the spring tides. The eggs hatch into free-swimming larvae that float in the plankton for up to six weeks. After settling and metamorphosing into the adult form, it takes a further two to three years for the adult to fully mature. It feeds mainly on algae, which it rasps from the rocks. In the 19th century, the common periwinkle was accidentally introduced into North America, where its selective grazing of fast-growing algal species has considerably affected the ecology of some rocky shores.
OCEAN LIFE
Coral reefs at about 50 ft (15 m)
body’s outer casing, or mantle. When threatened, however, its distinctive coloration quickly disappears as it withdraws all its soft body parts into its shell for protection. This snail feeds almost exclusively on gorgonian corals, which dominate Caribbean reef communities. Although these corals release chemical defenses to repulse predators, the flamingo tongue cowrie is apparently able to degrade these bioactive compounds and eat the corals without coming to any harm. After mating, the female strips part of a soft coral branch and deposits the egg capsules on it. Each capsule contains a single egg that will hatch into a free-swimming planktonic larva.
This gastropod is one of the very few animals that eats the crown-of-thorns starfish, itself a voracious predator and destroyer of coral reefs. The giant triton is an active hunter that will chase prey, such as starfish, mollusks, and sea stars, once it has detected them. It uses its muscular single foot to hold its victim down while it cuts through any protective covering using its serrated, tonguelike radula; it then releases paralyzing saliva into the body before eating the subdued prey.
286 CLASS GASTROPODA
CLASS GASTROPODA
Three-tooth Cavoline
Sea Hare
Cavolinia tridentata
Aplysia punctata
LENGTH
LENGTH
1/ 2
Up to 8 in (20 cm)
in (1 cm)
HABITAT
HABITAT
330–6,500 ft (100–2,000 m); carried in ocean currents DISTRIBUTION
CLASS GASTROPODA
Bubble Shell Bullina lineata LENGTH
1 in (2.5 cm) HABITAT
Sand, reefs to 65 ft (20 m), mainly intertidal; subtidal at range limits DISTRIBUTION Tropical and subtropical waters of Indian Ocean and west Pacific
The pale spiral shell of the bubble shell (also known as the red-lined bubble shell) has a distinctive pattern of pinkish red lines by which it can be
identified. Its soft body parts are delicate and translucent with a fluorescent blue margin and, in form, reminiscent of the Spanish dancer (opposite), which is a close relative that has lost its shell. If threatened, the bubble shell quickly withdraws into its shell and at the same time regurgitates food, possibly as a defense mechanism to distract predators. The bubble shell is itself a voracious predator, feeding on sedentary polychaete worms. This mollusk is hermaphroditic and produces characteristic spiral white egg masses.
Warm oceanic waters worldwide
This species of sea butterfly has a small, almost transparent, spherical shell with three distinctive, posterior projections. The shell also has two slits through which large extensions of the mantle pass. These brownish “wings” are ciliated and so can create weak water currents as well as aid buoyancy. Sea butterflies are unusual among shelled mollusks in that they can live in open water. Like other members of this group, the three-tooth cavoline produces a mucus web very much larger than itself, which traps planktonic organisms, such as diatoms and the larvae of other species. It eats the web and the trapped food at intervals, then produces a new one. During their lifetime, sea butterflies change first from males into hermaphrodites and then into females.
Shallow water
Northeast Atlantic and parts of the Mediterranean
DISTRIBUTION
The sea hare, a type of sea slug, has tentacles reminiscent of a hare’s ears. It has an internal shell about 11/2 in (4 cm) long that is visible only through a dorsal opening in the mantle. If disturbed, it releases purple or white ink. It is not known if this response is a defense mechanism.
CLASS GASTROPODA
CLASS GASTROPODA
Polybranchid
Hermissenda Sea Slug
Cyerce nigricans
Hermissenda crassicornis
LENGTH
Up to 11/2 in (4 cm)
LENGTH
HABITAT
Up to 3 in (8 cm)
Reefs
HABITAT
Mud flats, rocky shores DISTRIBUTION
Western Indian Ocean, western and
central Pacific
OCEAN LIFE
DISTRIBUTION
This colorful sea slug is a herbivore that browses on algae. It has no need of camouflage or a protective shell, as it has two excellent alternative defense strategies. First, it can secrete distasteful mucus, by utilizing substances in the algae it feeds on and secreting them from small microscopic glands over the body. Second, its body is covered with petal-like outgrowths called cerata, spotted and striped above and spotted below, that can be shed if it is attacked by a predator, in the same way as a lizard sheds its tail. This ability to cast off body parts to distract predators is called autotomy. The cerata are also used in respiration, their large collective surface area allowing efficient gas exchange with the surrounding water. The head carries two pairs of sensory organs— the oral tentacles near the mouth and, further back, the olfactory organs (rhinophores). These are retractile and subdivide as the polybranchid matures. They are used to assist in finding food and mates. There is some debate as to whether this sea slug is a separate species or is simply a color variation of a similar mollusk, Cyerce nigra.
CLASS GASTROPODA
Chromodorid Sea Slug Chromodoris lochi LENGTH
11/2 in (4 cm) HABITAT
Reefs
DISTRIBUTION
Tropical and subtropical western and
central Pacific
Protected from predators by its bright warning coloration and unpleasant taste, the chromodorid sea slug forages in the open, rather than hiding away
in cracks and crevices. Since it cannot swim, it glides over the tropical reefs on which it lives on its muscular foot, secreting a mucus trail much as terrestrial slugs do. The different species of the genus Chromodoris are distinguished by the pattern of black lines on their backs and the plain color of their gills and rhinophores (a pair of olfactory organs at the head end). The two chromodorid sea slugs pictured here are possibly about to mate. To do so, they must face in opposite directions so that their sexual openings are aligned. As they are hermaphrodites, they both produce sperm, which they exchange during mating, and both later produce fertilized eggs.
Northwest and northeast Pacific
This sea slug, usually known simply as Hermissenda, has an unusual way of deterring predators. It separates the stinging cells from any organism it eats and stores them in the orange-red tips of petal-like tentacles, or cerata, that cover its back. Any creature that touches the cerata is stung. Unlikely though it seems, Hermissenda is used extensively by scientists conducting memory experiments. The animal has an excellent sense of smell that enables it to find its way around mazes to locate food, and it can be “taught” to respond to simple stimuli.
MOLLUSKS CLASS GASTROPODA
Spanish Dancer Hexabranchus sanguineus LENGTH
Up to 24 in (60 cm)
exposing its bright colors and possibly startling potential predators. Spanish dancers are specialist predators that feed only on sponges, particularly encrusting species, from which they modify and concentrate certain distasteful compounds in their skin to
HABITAT
Shallow water on coasts and reefs
DISTRIBUTION
287
use as another defense against predation. They have external gills for respiration, which are extensively branched and attached to the body wall in distinct pockets and which cannot be retracted. Like all nudibranchs, the Spanish dancer is hermaphroditic, but it requires a partner in order to reproduce.
Parts of tropical Indian Ocean,
west Pacific
external gills
The largest of the nudibranchs is the Spanish dancer—so called because when it swims, the undulating movements of its flattened body are reminiscent of a flamenco dancer. Adults are brightly but variably colored, generally in shades of red, pink, or orange, sometimes mixed with white or yellow. While resting, crawling, or feeding, the lateral edges of its mantle are folded up over its back, displaying the less colorful underside. If disturbed, it will escape by swimming away,
SEA ROSE To protect its egg cluster from predators, the Spanish dancer deposits with its eggs some of the toxins that it produces for its own defense. Once hatched, the free-swimming larvae join the plankton until they mature. With a
EGG RIBBON
Each dancer produces several roselike pink egg ribbons about 1½ in (4 cm) across; together these may contain over one million eggs.
OCEAN LIFE
bright coloration
life-span of about a year, they grow rapidly, settling on a suitable food source when they are ready to change into the adult form.
288 CLASS CEPHALOPODA
CLASS CEPHALOPODA
Dumbo Octopus
Blue-ringed Octopus
Grimpoteuthis plena
Hapalochlaena maculosa
LENGTH
DISTRIBUTION
CLASS CEPHALOPODA
Nautilus Nautilus pompilius WIDTH
Shell up to 8 in (20 cm) HABITAT
Tropical open waters to 1,600 ft (500 m) DISTRIBUTION Eastern Indian Ocean, western Pacific, and Australia to New Caledonia
The five remaining species of Nautilus and Allonautilus belong to a once numerous group of shelled cephalopods that existed from 400 to 65 million years ago. They are often referred to as “living fossils” because they are so
CLASS CEPHALOPODA
Giant Octopus Enteroctopus dofleini LENGTH
Up to 15 ft (4.5 m) HABITAT
Bottom dwellers, 30–2,500 ft (9–750 m)
DISTRIBUTION
Temperate northwest and northeast
Pacific
OCEAN LIFE
The giant octopus is one of the largest invertebrates as well as one of the most intelligent. It can solve problems, such as negotiating a maze
little changed from their ammonoid ancestors.Their shell protects them from predation, while gas trapped in its inner chambers provides buoyancy. The head protrudes from the shell and has up to 90 suckerless tentacles, which are used to capture prey such as shrimp and other crustaceans; the head also features a pair of rudimentary eyes that lack a lens and work on a principle similar to a pinhole camera.The nautilus swims using jet propulsion, drawing water into its mantle cavity and expelling it forcefully through a tubular siphon, which can be directed to propel the nautilus forward, backward, or sideways. Unlike most other cephalopods, nautiluses mature late, at about ten years of age, and produce only about twelve eggs per year. by trial and error, and remember the solution for a long time. It has large, complex eyes with color vision and sensitive suckers that can distinguish between objects by touch alone. It changes color rapidly by contracting or expanding pigmented areas in cells called chromatophores, enabling it to remain camouflaged regardless of background. It also uses its color to convey mood, becoming red if annoyed and pale if stressed. Most cephalopods show little parental care, but female giant octopuses guard their eggs for up to eight months until they hatch. They do not eat during that time, and siphon water over the eggs to keep them clean and aerated.
LENGTH
Up to 8 in (20 cm)
4–8 in (10–20 cm)
HABITAT
HABITAT
Deep water, to 6,500 ft (2,000 m)
Shallow water, rock pools
Northwest Atlantic
Little is known about the Dumbo octopus, as only a few have been recorded. Its common name derives from a pair of unusual, earlike flaps extending from the mantle above its eyes. It has a soft body, an adaptation to its deep-water habitat, and eight arms connected to each other almost to their tips by “webbing.” Its diet includes worms and snails.
Tropical west Pacific and Indian Ocean (all species of Hapalochlaena)
DISTRIBUTION
The most dangerous cephalopod is the small blue-ringed octopus, which produces highly toxic saliva powerful enough to kill a human. To catch prey, it either releases saliva into the water and waits for the poison to take effect, or catches, bites, and injects prey directly. Its bright coloring is unusual for an octopus, and the numerous blue rings covering its body become more iridescent if it is disturbed.
DEFENSE MECHANISM When threatened, a giant octopus squirts a cloud of purple ink out through its siphon into the water and at the same time moves backward rapidly using jet propulsion. Potential predators are left confused and disoriented in a cloud of ink. The octopus can repeat this process several times in quick succession.
A QUICK GETAWAY
This giant octopus is making a rapid retreat, expelling an ink jet as a defense mechanism. The jet also propels the octopus backward forcefully.
MOLLUSKS
289
CLASS CEPHALOPODA
Australian Giant Cuttlefish Sepia apama LENGTH
Up to 5 ft (1.5 m) HABITAT
Shallow water over reefs
DISTRIBUTION
Coastal Australian waters
Of about 100 cuttlefish species, the Australian giant cuttlefish is the largest. Like all cuttlefish, it has a flattened body and an internal shell, known as the cuttle and familiar to many as budgerigar food. This species lives for up to three years and gathers in huge numbers to breed. Males have elaborate courtship displays, which involve hovering in the water while making rapid, kaleidoscopic changes of color, as the male shown here is doing. When a female is receptive, the male deposits a sperm package in a pouch under her mouth. This later bursts, releasing sperm and fertilizing her 200 or more golf-ball-sized eggs, which she then deposits on a hard substrate. The eggs hatch into miniature adults after several months.
CLASS CEPHALOPODA
CLASS CEPHALOPODA
Common Squid
Glass Squid
Loligo vulgaris
Teuthowenia pellucida
DISTRIBUTION
Vampire Squid
LENGTH
LENGTH
Up to 12 in (30 cm)
1/ –11/ in 2 2
HABITAT
HABITAT
60–800 ft (20–250 m)
Midwater
Eastern Atlantic, Mediterranean
DISTRIBUTION
CLASS CEPHALOPODA
Vampyroteuthis infernalis LENGTH
Circumglobal in southern temperate
waters
A tubular body and a small, rodlike internal skeleton are characteristic features of all species of squid. They also have very large eyes relative to body size. The common squid is an inshore, commercially important species that has been harvested for centuries and is probably the best known of all cephalopods. It is a fast swimmer that actively hunts its prey, such as crustaceans and small fish. Once caught, the squid passes the prey to its mouth, where it is dismembered by powerful, beaklike jaws.
Up to 15 in (38 cm)
(1.4–3.8 cm)
Like many mollusks, juvenile glass squid live in the plankton, then descend to deeper, darker levels as they mature. The presence of light organs, called photophores, in the tips of their arms and in the eye may help in locating a mate. Sexually mature females are also thought to produce a chemical attractant, or pheromone.
HABITAT
1,600–5,000 ft (500–1,500 m), oxygen-poor water DISTRIBUTION
Tropical and temperate oceans
worldwide
CLASS POLYPLACOPHORA
Lined Chiton Tonicella lineata LENGTH
11/2 in (3.5 cm) HABITAT
Intertidal and subtidal zones, common on rocky surfaces DISTRIBUTION Temperate waters of northeast and northwest Pacific
coralline algae. The lined chiton’s mantle extends around the shell on all sides, forming an unusually smooth, leathery “girdle” that helps to hold its eight shell-plates together. It has a large, muscular foot, which it uses to move over rocks and, when still, to grip on to them in much the same way as limpets do. At low tide, it remains stationary to avoid water loss. Its head is small and eyeless. The sexes are separate and it reproduces by releasing its gametes into the water.
OCEAN LIFE
Chitons are mollusks with shells made up of eight arching and overlapping plates. The lined chiton is so called because of a series of zigzagging blue or red lines on its shell. The shell is usually pinkish in color, which provides good camouflage as this chiton grazes from rocks that are covered with encrusting pink
This is the only squid that spends its entire life in deep, oxygen-poor water. Like many deep-living creatures, the vampire squid is bioluminescent and has light organs, or photophores, on the tips of its arms and at the base of its fins. If threatened, it flashes these lights and writhes around in the water, finally ejecting mucus that sparkles with blue luminescent light. When the lights go out, the vampire squid will have vanished. Its predators include deep-diving whales.
290
ANIMAL LIFE
Arthropods THE ANIMALS THAT HAVE ACHIEVED
the greatest diversity on Earth are the arthropods, of which insects are by far the most KINGDOM Animalia numerous. However, most marine arthropods are crustaceans, PHYLUM Arthropoda such as crabs, shrimp, and barnacles. Crustaceans, including SUBPHYLA 4 both adults and larval stages, form most of the ocean’s SPECIES About 1.25 million zooplankton—the community of tiny, drifting life forms that support all oceanic food chains. Like land arthropods, all marine forms have an external skeleton, segmented body, and jointed appendages, permitting some, such as robber crabs, to live on land as well as in water. Fully marine insects are rare but some live on seashores and coasts. DOMAIN Eucarya
APPENDAGES
Anatomy
SEASHORE INSECTS
Springtails are wingless relatives of true insects. Marine species can be found in the upper regions of seashores and will spring into the air if disturbed. digestive gland
heart
This spotted cleaner shrimp has jointed walking appendages. Two furthers pairs of jointed appendages, which are located on its head, are modified into sensory antennae.
Although arthropods may look very different from one another, they all have an external skeleton (exoskeleton), which is either thin and flexible or rigid and toughened by deposits of calcium carbonate. The body is segmented and has a variable number of jointed appendages—some are used for walking and swimming, while others are modified into claws and antennae or adapted for feeding. Muscles are attached across the joints to facilitate movement. Most of the body cavity is hollow; this space, called the hemocoel, contains the internal organs and a fluid—hemolymph—that is the equivalent to vertebrate blood, which is pumped around the body by the heart in an open circulatory system. Most marine forms use gills for respiration and have well-developed sense organs. sensory antenna
stomach
merus
SPIDER FEATURES
Although it is called a crab and has a hinged carapace, this horseshoe crab is a close relative of spiders, ticks, and mites. Like them, it has piercing mouthparts.
dactylus
carpus eye
propodus
claw ischium coxa basis
swimmeret tail fan (telson)
nerve cluster (ganglion)
forward and backward movement
point of attachment to body
movements up and down
ARTHROPOD ANATOMY
ARTHROPOD LIMB
Lobsters have a protective shell carapace covering the head and thorax, large pincers, and well-developed walking appendages. The hemocoel contains the internal organs.
Walking appendages, such as this crab’s leg, comprise rigid sections linked with movable joints. The joints move in different planes, allowing versatile movement.
FILTER-FEEDING LIMBS
At high tide, barnacles feed by extending their long, feathery appendages from their “shell” and sweeping the water for plankton and detritus.
OCEAN LIFE
walking appendage
SCAVENGING IN THE SAND
As the tide retreats, this sand bubbler crab emerges to feed on the organic material contained in the sand.
Feeding Among crustaceans, feeding is extremely varied. Many crabs are scavengers that feed on dead and decaying organic matter. They are therefore vital in helping to recycle nutrients. Others are hunters and have robust claws to stun (mantis shrimp) or crush (lobsters) their prey before tearing it apart and consuming it. Many small planktonic crustaceans, such as copepods, are filter feeders that make effective use of various appendages, including long antennae, to create water currents that waft food particles toward their mouths. Barnacles, which are attached to rocks and unable to move, feed in a similar way, using their limbs to collect plankton. A few crustaceans are parasitic (some isopods, copepods, and the barnacle Sacculina) and obtain all their nourishment from their host. The shoreline is an ideal place for insects, such as kelp flies, that feed by releasing enzymes onto rotting seaweed and then taking in the resultant digested material. Farther inland, among the dunes, there is more vegetation, so spiders and pollen- and nectar-feeding insects start to appear.
HARD SKELETON AND JOINTED LIMBS
These tiny porcelain crabs, less than 1 in (2.5 cm) wide, are filter feeders. They have typical arthropod features, such as a hard exoskeleton covering a segmented body, and jointed limbs.
ARTHROPODS
Growth
291
HUMAN IMPACT
KRILL DECLINE
Crustaceans can only develop and grow by molting and replacing their exoskeleton with a larger one. The moulting process, called ecdysis, is controlled by hormones and occurs repeatedly during adult life. The exoskeleton is produced from the layer of cells situated immediately below it. Before a molt starts, the exoskeleton detaches from this cell layer and the space in between fills with molting fluid. Enzymes within this fluid weaken the exoskeleton so that it eventually splits at the weakest point, often somewhere along the back. The new exoskeleton is soft and wrinkled, so it needs to expand and then harden. Marine arthropods absorb water rapidly after molting to expand their new protective covering. Those that can remain hidden for hours or days, as they are more vulnerable to predation until their exoskeleton hardens.
Antarctic krill are only 21⁄2 in (6 cm) long but are among the most abundant crustacean arthropods. Numbers in the Southern Ocean have fallen over the past few decades, partly due to rising water temperatures and melting of ice. Krill feed on algae that grow beneath and within the sea ice, and shelter under the ice to avoid predation. Over-harvesting of krill for human and animal feed also poses a significant threat with potential to disrupt the Antarctic food web (see p.295).
THE MOLTING SEQUENCE
This sequence shows a harlequin shrimp molting. The exoskeleton has split just behind the neck joint, allowing the shrimp to pull out its head. The rest of its body quickly follows as the split enlarges. It takes only a few minutes for the shrimp to free itself completely, after which it rests for a few seconds. The new exoskeleton is soft, since it must be flexible to buckle up to fit inside the older, smaller skeleton. It stretches to accommodate the increased size of the shrimp. Complete hardening of the new exoskeleton will take about two days.
The old exoskeleton splits along the back behind the harlequin shrimp’s head. It eases out backward.
1
The shrimp emerges further and struggles to free itself from the old exoskeleton.
2
Molting is complete, and the old exoskeleton lies beside the shrimp, as the animal rests.
3
OCEAN LIFE
292
Lifestyles
PARASITISM
SEA SPIDERS As a group, sea spiders are typical of many problematic organisms whose classification is changing as information becomes available. In line with current thinking, they form a class (pycnogonids) within the subphylum that also contains spiders and horseshoe crabs (chelicerates). However, continuing research indicates they may form a completely separate group of arthropods. DECEPTIVE APPEARANCE
Sea spiders are so called because of their resemblance to land spiders, but their exact relationship to spiders is still not clear.
Some arthropods live closely with other species. This fish is being parasitized by an isopod, which is related to woodlice. There are two isopods, one under each eye, feeding on tissue fluid to the detriment of the fish.
All marine arthropods are freeliving for at least part of their lives. Some, such as crabs, have planktonic larvae that sink to the sea floor and become bottomliving, or benthic, as they mature. Crabs and their relatives tend to live alone unless seeking a partner to breed with, and may defend COMMENSALISM territory. Others, such as This crab is camouflaged by a sea squirt in krill and copepods, live in a commensal relationship. The crab benefits but the sea squirt neither gains nor loses. vast swarms, traveling hundreds of yards up and down the water column each day to feed (see p.221). Adult barnacles remain anchored to one spot (that is, they are sessile), often settling in large aggregations on rocky shores where living conditions are most favorable. Deep-sea arthropod species are not well known, but many have cryptic red or black coloration to make themselves invisible, while others such as krill, have light organs and exhibit bioluminescence. A few arthropods live in close association with other species. Sometimes both partners benefit from a relationship (mutualism), sometimes only one has an advantage (commensalism), and sometimes one gains at some cost to the other (parasitism).
Reproduction and Life Cycles
egg mass on underside of female crab
In most crustaceans, the sexes are separate, fertilization is often internal, and the eggs must be laid in water. Some females store sperm and then let it flow over their eggs as they release them. Others protect their eggs by carrying them around, and keep them healthy by continually wafting water over them. On hatching, the larvae join the zooplankton, and pass through various stages before maturing into adults. Barnacles are both male and female (hermaphroditic) but only function as one sex at a time. The male has a long, extendable penis and mates with all neighboring females within reach. In horseshoe crabs, fertilization occurs externally. Males and females pair up, the males fertilize the eggs as the females lay them in the sand, and then both sexes abandon them.
ARTHROPOD CLASSIFICATION Arthropods are split into four subphyla—Crustacea, Chelicerata, Hexapoda, and Myriapoda. All have marine species except for the non-marine Myriapoda (centipedes and millipedes, not described below). CRUSTACEANS Subphylum Crustacea
OCEAN LIFE
61,710 species
Crustaceans are the dominant marine arthropod group and include the familiar crabs, lobsters, shrimps, prawns, and barnacles as well as the smaller copepods, isopods, and krill. Most crustaceans have two pairs of antennae and three body segments, the head and the thorax (often fused together as the cephalothorax) and the abdomen. The head and thorax are often protected by a shield, or carapace. Paired appendages vary
greatly—some are sensory, while others are adapted for walking or swimming; sometimes there is also a large pair of claws. CHELICERATES Subphylum Chelicerata 71,004 species
Spiders, scorpions, ticks, mites, horseshoe, crabs, and sea spiders belong to this group. A few species of spiders live in the intertidal zone, and some types of ticks and mites are either free-living or parasitic in marine habitats. The horseshoe crabs (class Merostomata) are completely marine. They have five pairs of
legs and their body comprises two parts, called the prosoma and opisthosoma. In horseshoe crabs, the prosoma contains most of the body organs, and the opisthosoma has most of the musculature and the book gills, which are used for respiration. What makes this group unique among chelicerates is the hinged carapace that protects the body and the long, tail-like telson, which the crab uses to right itself if it is accidentally inverted. Sea spiders (class Pycnogonida) are all marine. They are spiderlike, and most have a leg span of less than 1 in (2.5 cm). Many species have a unique pair of appendages, called ovigers, overhanging the head. The females use them for grooming, courtship, and also to transfer eggs to the ovigers of the male, where they remain until they hatch. Sea spiders are common in intertidal areas, but they are rarely seen due to their small size.
CRAB MOTHER AND LARVA
A velvet crab (above) carries eggs beneath her body until they hatch. The hatchlings enter a planktonic larval stage called a zoea (left). This molts four to seven times before it becomes a megalops larva, then once again to become an adult crab.
HEXAPODS Subphylum Hexapoda 1.11 million species
By far the largest group within the Hexapoda is the insects—the largest of all animal groups. It includes beetles, flies, ants, and bees. Most insects have compound eyes and three distinct body segments—the head, the thorax with its three pairs of walking appendages, and the abdomen. Many species also have wings. Many insects live in coastal areas, but only a few live on the shore. Only one type of insect is truly marine—the marine skater, Halobates, a type of “true bug” (order Hemiptera). Of the 40 coastal species, only five are able to spend their entire life on the ocean. However, they require a solid object, such as a floating feather or lump of tar, on which to lay their eggs.
ARTHROPODS SUBPHYLUM CHELICERATA
SUBPHYLUM CHELICERATA
Giant Sea Spider
American Horseshoe Crab
Colossendeis australis 10 in ( 25 cm) (leg-span)
LENGTH
WEIGHT
Not recorded
HABITAT
Bottom
Limulus polyphemus LENGTH
Up to 24 in
(60 cm) WEIGHT
dweller
Up to 11 lb (5 kg)
Sandy or muddy bays to 100 ft (30 m) HABITAT
DISTRIBUTION
Antarctic shelf and slope
Unlike most sea spiders, which have a leg-span of less than 1 in (2.5 cm), the giant sea spider has a huge leg-span of about 10 in (25 cm). It has a large proboscis through which it sucks its food, but its body is so small that the sex organs and parts of its digestive system are situated in the tops of the legs. Sea spiders are somewhat unusual among arthropods in that some exhibit parental care, the males having a modified pair of legs to carry the eggs until they hatch.
Western Atlantic and Gulf Coast from southern Maine to the Yucatán Peninsula
DISTRIBUTION
Despite its name, the American horseshoe crab is more closely related to spiders than to crabs. It is mainly active at night and scavenges anything it can find, including small worms, bivalves, and algae. Its horseshoeshaped, greenish-brown outer shell, or carapace, is for protection, and adults have few predators. It has six pairs of thoracic appendages: the first pair (called chelicerae), is used for feeding;
the other five are for walking and for grasping and tearing food. Five pairs of flattened abdominal appendages are used for swimming and scuttling along the bottom and also carry pagelike “book gills,” through which oxygen is absorbed. Its long, rigid tail is used for steering and for righting itself. Hinged to the body, it can act as a lever. The reproductive cycle is closely linked to spring high tides, when adults gather in large numbers on sandy beaches to breed. Females lay up to 80,000 eggs in a depression near the high-tide mark, providing a vital food source for birds and other marine creatures. The eggs are laid in batches mostly when the moon is in its full or new phases. The eggs hatch into tailless “trilobite larvae,” so called because they look a bit like fossil trilobites. Carried down the shore at high tide, the larvae swim around actively but also burrow into the sediment for safety. After a few days, they molt and become juveniles.
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HUMAN IMPACT
MEDICAL RESEARCH If the American Horseshoe Crab is injured, some of its blood cells form a clot, which kills harmful negative bacteria. In order to exploit this property for human benefit, crabs are collected from shallow waters on the Atlantic coast of North America during the summer months. Researchers then remove about 20 percent of the blood from each crab. From this they extract a protein that is used to detect bacterial contamination in drugs and medical devices that will be in contact with blood. Taking blood from the crabs is sometimes fatal to them but most recover after they have been returned to the sea.
SUBPHYLUM CRUSTACEA
Water Flea Evadne nordmanni LENGTH 1/32 in WEIGHT
(1 mm)
Not recorded
Open waters, to depths of 6,500 ft (2,000 m)
HABITAT
DISTRIBUTION
Temperate and cool waters worldwide
Most water fleas live in freshwater but this species and just a few others live as plankton in the ocean. It feeds on tiny bacteria, protists, and organic debris and is eaten by larger planktonic animals. It has a single conspicuous eye and feathery swimming appendages that are modified antennae. Females brood unfertilised eggs that hatch into more females. Sexual reproduction also occurs. large eye
OCEAN LIFE
feathery swimming appendage
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ANIMAL LIFE SUBPHYLUM CRUSTACEA
Cyclopoid Copepod
Gooseneck Barnacle
Oithona similis
Pollicipes polymerus LENGTH 1/ –3/ in 54 32
LENGTH
(0.5–2.5 mm)
Up to 3 in (8 cm)
HABITAT
HABITAT
Surface waters to a depth of 500 ft (150 m)
Intertidal zone of rocky shores
DISTRIBUTION Atlantic, Mediterranean, Southern Ocean, southern Indian and Pacific oceans
DISTRIBUTION Eastern Pacific coast of North America, from Canada to Baja California, Mexico
Copepods make up a large percentage of zooplankton, and this is one of the most abundant, widespread species. As the name suggests, cyclopoid copepods have a single, central eye, which is light sensitive. They have a tiny, oval body that tapers to a thin tail. Most have six body segments and six pairs of swimming limbs. Tiny food particles are filtered from the water using specialised mouthparts. Females can be recognized when carrying egg sacs attached to their abdomens. As part of the zooplankton, copepods of this genus are a vital element of oceanic food chains. They feed on marine algae and bacteria and in turn are an important source of protein for many ocean-dwelling animals. Every night cyclopoid copepods migrate from a depth of about 500 ft (150 m) to the surface layers of the ocean to feed. This daily journey, which is undertaken by many marine creatures, is one of the largest mass movements of animals on Earth.
So called because of its resemblance to a goose neck and head, the gooseneck barnacle forms dense colonies in crevices on rocky shores with strong waves. Barnacles anchor themselves to rocks by a tough, flexible stalk (peduncle), which also contains the gonads. This is actually their “head” end. Once the barnacle has attached itself to an object it secretes a series of pale plates at the end of its stalk, forming a shell around its featherlike legs, which comb through the water for food. The legs face away from the sea, enabling the barnacle to feed by filtering out particles of detritus from returning tidal water as it funnels past them through cracks in the rocks. These barnacles become sexually mature at about five years of age and may live for up to 20 years. The larval stage is free-living but depends on sea currents for its transport and survival. Colonies of gooseneck barnacle are susceptible to the damaging effects of oil pollution and they recover only slowly from disturbance.
SUBPHYLUM CRUSTACEA
Acorn Barnacle Semibalanus balanoides LENGTH
Up to 1/2 in (1.5 cm) diameter HABITAT
Intertidal zone of rocky shores DISTRIBUTION Northwest and northeast Atlantic, Pacific coast of North America
OCEAN LIFE
SUBPHYLUM CRUSTACEA
Like all adult barnacles, the adult acorn barnacle remains fixed in one place once it has anchored itself to a site. The free-swimming juveniles pass through several larval stages before molting into a form that can detect both other acorn barnacles and suitable anchoring sites. Once a larva fixes itself to a rock, using cement produced by glands in its antennae, it molts again. It then secretes six gray calcareous plates, forming a protective cone that looks rather like a miniature volcano. Four smaller, movable plates
at the top of the cone open, allowing the acorn barnacle to feed. It does this when the tide is in by waving its modified legs, called cirri, in the water to filter out food. When the tide is out, the plates are closed to prevent the barnacle from drying out. Acorn barnacles are hermaphrodites that possess both male and female sexual organs, but they function as either a male or a female. They do not shed their eggs and sperm into the water; instead they use extendable penises, to transfer sperm to receptive neighbors.
PEOPLE
CHARLES DARWIN Before the British naturalist Charles Darwin (1809–1882) proposed his revolutionary theory of evolution in The Origin of Species (1859), he spent eight years studying barnacles. Realizing the impact his ideas on evolution would have on existing scientific and religious thinking, he delayed writing and instead produced four monographs on the classification and biology of barnacles. This work earned him the Royal Society’s Royal Medal in 1853, validating his reputation as a biologist.
ARTHROPODS SUBPHYLUM CRUSTACEA
Giant Mussel Shrimp Gigantocypris muelleri LENGTH 1/ –3/ in 2 4
(1.4–1.8 cm)
SUBPHYLUM CRUSTACEA
Peacock Mantis Shrimp Odontodactylus scyallarus LENGTH
HABITAT
Up to 6 in (15 cm)
Planktonic, intermediate to deep sea
DISTRIBUTION
HABITAT
Warm water near reefs with sandy, gravelly, or shelly bottoms
Atlantic, Southern Ocean, western
Indian Ocean DISTRIBUTION
Indian and Pacific oceans
Like all ostracod crustaceans, the giant mussel shrimp has a carapace that encloses its body, so that its seven pairs of limbs are almost hidden from view. Its large, mirror eyes with parabolicshaped reflectors focus light on to a flat plate in its center. It is planktonic, but lives at greater depth than many forms of plankton, usually below 650 ft (200 m), where it feeds on falling detritus. The picture shows a large female carrying embryos, which are clearly visible through the carapace.
SUBPHYLUM CRUSTACEA
Sea Slater Ligia oceanica LENGTH
Up to 11/4 in (3 cm) HABITAT
Coasts with rocky substrata
DISTRIBUTION
Atlantic coasts of northwestern
Europe
Commonly found under stones and in rock crevices, the sea slater is a seashore-dwelling a member of the isopods (a group that also includes woodlice). It lives in the splash zone, but can survive periods of immersion in salt water. Its head, which has a pair of well-developed compound eyes and very long antennae, is
SUBPHYLUM CRUSTACEA
Sand Hopper Orchestia gammarellus 1/16 –3/8 in
(2–10 mm)
HABITAT
Splash zone of sandy shores DISTRIBUTION Atlantic coasts of northeastern Canada and northwestern Europe
uropod
SUBPHYLUM CRUSTACEA
Antarctic Krill Euphausia superba LENGTH
Up to 2 in (5 cm) HABITAT
Planktonic
DISTRIBUTION
Southern Ocean
All oceans contain krill—small, shrimplike, planktonic crustaceans that live in open waters. Antarctic krill live in vast numbers in the subantarctic waters of the Southern
Such power is created by a special, saddlelike hinge-joint in these legs, which acts like a spring. The peacock mantis shrimp can smash the shells of gastropods and crabs and tackles prey larger than itself. It excavates U-shaped burrows or lives in crevices in rocks or coral. After hatching, its larvae enter the plankton, where they develop over a few weeks before drifting down toward the sea floor to make their own homes.
Ocean, where they form a vital link in the food chain, being eaten in vast quantities by baleen whales, seals, and various fish. Krill rise to the surface at night to feed on phytoplankton, algae, and diatoms. For safety they sink to greater depths during the day. The feathery appearance of this species is due to its gills, which, unusually, are carried outside the carapace. Their filamentous structure increases the surface area available for gaseous exchange. Antarctic krill also have large light organs, called photophores. The light is thought to help them group together. They spawn in spring, during which females may release several broods of up to 8,000 eggs.
antenna
Amphipod crustaceans, such as the sand hopper, live in large numbers in the splash zone of any shore where there is rotting seaweed. Their life cycle takes about 12 months and the female usually produces only one clutch of eggs, which she keeps in a brood pouch, where they hatch after one to three weeks. The young leave the pouch about a week later when their mother molts. Sand hoppers are also known as sand fleas because they jump about in a similar way and have similar laterally compressed bodies.
OCEAN LIFE
LENGTH
not markedly separated from its body, which is flattened, about twice as long as it is broad, and ends in two forked projections called uropods. As adults, sea slaters have six pairs of walking legs until their final molt, after which they have seven. The sea slater is not generally seen during the day unless it is disturbed, and it emerges from its hiding place only at night to feed on detritus and decaying seaweed. Sea slaters mature at about two years of age and usually breed only once before dying.
The brightly colored peacock mantis shrimp is a stomatopod crustacean. Like all members of this group, it is a voracious predator. Its large, mobile, compound eyes have sophisticated stereoscopic and color vision that includes some ultraviolet shades. It uses sight when hunting, waiting quietly, like the praying mantis, for its unsuspecting prey to come within reach, then striking using its powerful, clublike second pair of legs with immense speed—about 75 mph (120 km/h)—and force (up 100 times its own weight).
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ANIMAL LIFE SUBPHYLUM CRUSTACEA
SUBPHYLUM CRUSTACEA
Deep Sea Red Prawn
Common Prawn
Acanthephyra pelagica
Palaemon serratus
LENGTH
DISTRIBUTION
41/4 in
Up to
HABITAT
HABITAT
Atlantic
In the low light levels of the deep ocean, red appears black, making the deep sea red prawn invisible to potential predators. Its hard outer casing, or exoskeleton, is thinner and more flexible than that of shallowwater crustaceans, which helps prevent them from sinking into the depths. The flesh of this prawn is also oily to aid further buoyancy. It uses its first three pairs of limbs to feed on small copepods. The remaining five pairs of limbs, the pereiopods, are used for locomotion. Gills attached to the tops of the legs are used for respiration.
PRAWN FARMING
LENGTH
Not recorded Deep water
HUMAN IMPACT
(11 cm)
Rock pools, shallow water, and lower parts of estuaries DISTRIBUTION Eastern Atlantic from Denmark to Mauritania, Mediterranean, Black Sea
The common prawn has a semitransparent body, making its internal organs visible, and is marked with darker bands and spots of brownish red. As with many other species of prawn, its shell extends forward between its stalked eyes to form a stiff, slightly upturned projection called a rostrum. This feature has a unique structure by which the common prawn can be distinguished from all other members of the same genus. The rostrum curves upward, splitting in two at the tip, where it has several toothlike projections on the lower and upper surfaces. To either side of the rostrum there is a very long antenna that can sense any danger close by and is also used to detect food. Of the prawn’s five pairs of legs, the rear three pairs are used for walking, while the front two pairs are pincered and used for eating. Attached to the abdomen is a series of smaller limbs called swimmerets that the
Nearly all the world’s farmed prawns come from developing countries such as Thailand, China, Brazil, Bangladesh, and Ecuador, which use intensive farming to meet demand. Cutting mangrove forests to construct prawn ponds is now being discouraged. SHARED RESOURCES
Fishermen in Honduras fish for wild prawns in a lagoon shared with prawn farmers.
prawn uses to swim. For a sudden, backward movement, the prawn flicks its tail. Females produce and look after about 4,000 eggs until they hatch into larvae. The larvae float among the plankton until they mature. pincered leg
tail fan, or telson
SUBPHYLUM CRUSTACEA
Anemone Shrimp Periclimenes brevicarpalis LENGTH
1 in (2.5 cm) HABITAT
Shallow water reefs
OCEAN LIFE
DISTRIBUTION
Indian Ocean, western Pacific
Nestling among the tentacles of an anemone, the anemone shrimp is safe from attack by predators. It rarely wanders far from its host, surviving by scavenging scraps that the anemone cannot eat. The shrimp may benefit the anemone by removing excess food particles as well as any waste it produces. This type of relationship is called commensalism: one individual in the partnership profits from the liaison and the other comes to no harm. Removed from its host, this shrimp is defenseless. The anemone shrimp belongs to the same family (Palaeomonidae) as the common prawn (above) and so they have several features in common. These include a pair of long, sensory antennae used to sense danger and detect food and a rostrum (the elongated projection of the shell from between the eyes). The anemone shrimp is almost completely transparent, with a few purple and white spots.
ARTHROPODS SUBPHYLUM CRUSTACEA
SUBPHYLUM CRUSTACEA
Spiny Lobster
European Lobster
Panulirus argus
Homarus gammarus LENGTH
LENGTH
24 in (60 cm)
Up to 3 ft (1 m), typically 24 in (60 cm)
HABITAT
Coral reefs and rocky areas
DISTRIBUTION
HABITAT
Rocky coasts
Western Atlantic, Gulf of Mexico,
DISTRIBUTION Eastern Atlantic, North Sea, Mediterranean
Caribbean Sea
Being both nocturnal and migratory, the spiny lobster has excellent navigational skills. It can establish its position in relation to Earth’s magnetic field and then follow a particular route as well as any homing pigeon. This lobster prefers warm
SUBPHYLUM CRUSTACEA
Reef Hermit Crab Dardanus megistos WIDTH (LEG-SPAN)
Up to 12 in (30 cm) HABITAT
Near-shore tropical reefs
DISTRIBUTION
Indian and Pacific oceans
Like other hermit crabs, the reef hermit crab uses an empty gastropod mollusc shell to protect its soft abdomen. When it grows too big for its current shell, it simply looks for an unused larger one or evicts a weaker rival. It is while switching from one
SUBPHYLUM CRUSTACEA
Robber Crab Birgus latro LENGTH
Up to 24 in (60 cm) across HABITAT
Rock crevices and sandy burrows
DISTRIBUTION
oceans
Tropical waters of Indian and Pacific
water and so remains in the shallows in summer before migrating in groups to deeper water in winter by walking in single file across the sea floor. It lacks the large claws of the European lobster (right) but is well protected from most predators by the sharp spines that cover its carapace. shell to the next that the reef hermit crab is most vulnerable, as it risks exposing its soft, rather asymmetrical abdomen to predators. There are about 1,150 species of hermit crabs — the reef hermit crab lives in shallow-water tropical reef habitats, but some species live on coastal land. The reef hermit crab is a scavenger rather than a hunter, and drags itself over the seafloor looking for bits of animal matter and algae, tearing apart any carcasses that it finds with its dextrous mouthparts. Its close relative Dardanus pedunculatus attaches stinging anemones to its shell as rotection from predators. The robber crab, or coconut crab, is the largest terrestrial arthropod. Like its close relatives other hermit crabs, it lives inside a mollusk shell when young but discards this as it grows bigger and tougher. It lives on oceanic islands and offshore inlets. It mainly eats fruit and nuts but will also scavenge for carrion. It can smash and eat coconuts but rarely does so. Adults live and mate on land, but females release their eggs into water.
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In life the European lobster is brown or bluish—it only turns the familiar red when it is cooked. Individuals weigh up to 11 lb (5 kg). It has large, differently sized claws: the smaller one has sharper edges and is used for cutting prey, while the larger one is used for crushing. The European lobster lives in holes and crevices on the sea bed. The European lobster is commercially important and in danger of overexploitation because it matures slowly, and is such a valuable commodity.
crushing claw cutting claw
eye stalk
SUBPHYLUM CRUSTACEA
Porcelain Crab single large claw
Petrolisthes lamarckii WIDTH (SHELL)
Up to 3/4 in (2 cm) HABITAT
Pools on rocky beaches and shorelines DISTRIBUTION Indian Ocean, Pacific coast of Australia, western Pacific
SUBPHYLUM CRUSTACEA
The flat, rounded body of the porcelain crab allows it to slip easily into small rock crevices to hide. However, if it becomes trapped by a predator or stuck beneath a rock, it can shed one of its claws in order to escape, and a new one will grow over time. This crab’s abdomen is folded under its body, but it can be unfolded and moved like a paddle when swimming.
Nodose Box Crab Cyclozodion angustum LENGTH
Not recorded HABITAT
Offshore to depths of 50–650 ft (15–200 m)
DISTRIBUTION
Western Atlantic, Gulf of Mexico,
Caribbean Sea
Previously known as Calappa angusta, the nodose box crab is a true crab with a small abdomen that is tucked away underneath the body and four pairs of legs. This species may be recognized by the rows of nodules that radiate from behind its eyes across the upper surface of its yellowish shell, or carapace.
SUBPHYLUM CRUSTACEA
Japanese Spider Crab Macrocheira kaempferi WIDTH (SHELL)
Up to
141/2 in (37 cm) Deep-water vents and holes to depths of 160–1,000 ft (50–300 m)
HABITAT
DISTRIBUTION
Pacific Ocean near Japan
OCEAN LIFE
Not only is the giant Japanese spider crab the largest of all crabs, with a leg-span of up to 13 ft (4 m) and weighing 35–44 lb (16–20 kg), it may also be the longest living, estimated to live for up to 100 years. Living in the deep, cold waters around Japan, it moves slowly across the ocean floor on its spiderlike legs, scavenging for food.
PORCELAIN CRAB
The porcelain crab uses its flat body to crawl out of reach of predators. Here, the tentacles of an anemone provide a secure and permanent home for a porcelain crab in the Andaman Sea in the northern Indian Ocean. The mouthparts of the crab fan out and trap plankton, which it then brushes into its mouth.
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ANIMAL LIFE SUBPHYLUM CRUSTACEA
Long-legged Spider Crab Macropodia rostrata LENGTH
Up to 1 in (2.5 cm) HABITAT
Lower shore, usually not beyond 165 ft (50 m) Northeastern Atlantic from southern Norway to Morocco, Mediterranean DISTRIBUTION
Also called the decorator crab because it camouflages itself using fragments of seaweed and sponges, the longlegged spider crab is covered in
SUBPHYLUM CRUSTACEA
hook-shaped hairs that hold its disguise in place, enabling it to blend in with the seaweed among which it lives. This crab has a triangular-shaped carapace that extends forward between the eyes into an eighttoothed projection called a rostrum. Its spiderlike legs are at least twice as long as its body and can be used, somewhat ineffectively, for swimming. The long-legged spider crab feeds on small shellfish, algae, small worms, and detritus. Breeding occurs year-round on Atlantic coasts, but takes place between March and September in the Mediterranean. The male transfers sperm to the female using its first pair of abdominal legs. The female carries the eggs until they hatch into larvae that live in the plankton.
SUBPHYLUM CRUSTACEA
Edible Crab
Pea Crab
Cancer pagurus
Pinnotheres pisum LENGTH
LENGTH
Up to 6 in (16 cm)
Males 1/3 in (8 mm); females 1/2 in (14 mm)
HABITAT
Intertidal zone to 330 ft (100 m), in rock pools and muddy sand offshore
SUBPHYLUM CRUSTACEA
Spotted Reef Crab Carpilius maculatus LENGTH
About 31/2 in (9 cm) HABITAT
Shoreline to 33 ft (10 m), inshore reefs
DISTRIBUTION
SUBPHYLUM CRUSTACEA
Common Shore Crab Carcinus maenas LENGTH
Up to 21/2 in (6 cm)
HABITAT
Intertidal zone to 500 ft (150 m)
DISTRIBUTION Northeastern Atlantic and North Sea; introduced to parts of the Mediterranean
DISTRIBUTION Eastern Atlantic from northwestern Europe to West Africa, Mediterranean
The oval carapace of the edible crab has a characteristically “scalloped” or “piecrust” edge around the front and sides. Its huge pincers are distinctively black-tipped, while the body is purple-brown in small individuals and reddish brown in larger ones. Edible crabs mate at different times in different parts of their range, the females incubating their eggs for six to nine months. This crab is caught in large numbers and is highly valued as a luxury food.
Typically about the size of a pea, the tiny pea crab is usually found inside the shells of the common mussel. Protected from predators in the mantle cavity of its host, it feeds on any plankton that becomes trapped on the mussel’s gills as water passes over them. Whether the presence of this guest is harmful to the mussel is unclear. Female pea crabs are substantially larger than males and have an almost translucent carapace through which their pink reproductive organs are visible. Males have harder, yellowish brown carapaces that protect them during the breeding season, which runs from April to October. During this time, males leave the safety of their host’s shell and swim around looking for females with which to mate. In regions where shellfish are harvested commercially, the pea crab is considered a pest.
HABITAT
Intertidal zone to 200 ft (60 m), all substrates; estuaries Northeastern Atlantic from Norway to West Africa; introduced elsewhere
DISTRIBUTION
black eye on short eye stalk
mouth
Indian Ocean and western Pacific
The conspicuous coloring of the spotted reef or coral crab is highly distinctive. Its smooth, light brown carapace has two large red spots behind each eye, three across the middle, and either two or four at the rear. Between the eyes the carapace has four small, rounded projections, which are also characteristic of the species. It is a nocturnal, slow-moving crab that uses its disproportionately large claws to feed on corals, snails, and other small marine creatures.
The common shore crab tolerates a wide range of salt concentrations and temperatures and so can live in salt marshes and estuaries as well as along the shoreline. Its dark green carapace has five marked serrations on the edge behind the eyes. This opportunistic hunter preys voraciously on many types of animals, including bivalve mollusks, polychaetes, jellyfish, and small crustaceans. Where introduced, it may be detrimental to local marine life. On the west coast of the US, for example, it has had a considerable impact on the shellfish industry.
ARTHROPODS SUBPHYLUM CRUSTACEA
Blue Swimming Crab Portunus pelagicus LENGTH
Up to 23/4 in (7 cm) HABITAT
Intertidal sandy or muddy sea beds to 180 ft (55 m) DISTRIBUTION Coastal waters of the Indian and Pacific oceans, eastern Mediterranean
SUBPHYLUM CRUSTACEA
Orange Fiddler Crab Uca vocans LENGTH
About 1 in (2.5 cm) HABITAT
Near water on mud or sand
DISTRIBUTION
Indian Ocean and western Pacific
Like its close relative the ghost crab (see above, right), the male orange fiddler crab also exhibits ritualistic displays to deter rivals. Males are easily recognized because one of their claws
Unlike most crabs, the blue swimming crab is an excellent swimmer and uses its fourth pair of flattened, paddlelike legs to propel itself through the water. Despite its common name, only the males are blue, and the females are a rather dingy greenish brown. Males also differ in having very long claws, more than twice as long as the width of their carapace. The claws are armed with sharp teeth that are used to snag small fish and other food items. When a blue swimming crab feels threatened, it usually buries itself in the sand. If this measure fails to deter the threat, the crab adopts its own threat stance, extending its claws sideways in an attempt to look as large as possible. The natural range of this crab has been extended to a small part of the eastern Mediterranean by the opening of the Suez Canal. It is a popular food in Australia. is greatly enlarged. In a mature adult this claw makes up more than half the crab’s body weight and is used both to attract potential mates and to ward off rival males. Observing the distinctive “courtship wave” of fiddler crabs is helpful in identifying different species. Orange fiddler crabs are active during the day. As well as digging a main burrow up to 12 in (30 cm) deep, they create a number of bolt holes into which they can retreat if danger threatens. At high tide, they seal themselves into their burrows with a small pocket of air. The presence of air is essential for their survival because fiddler crabs obtain oxygen from air, not water, despite having gills.
SUBPHYLUM CRUSTACEA
Ghost Crab Ocypode saratan LENGTH
About 11/2 in (3.5 cm) HABITAT
Sandy shores in deep burrows above the water line DISTRIBUTION Coastal waters of western Indian Ocean, Red Sea
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Resting in the cool of its burrow during the day, the fast-moving ghost crab emerges at twilight to hunt. It will eat anything it can find, including other crabs, and also scavenges whatever was brought in by the last tide. During the mating season, males defend their burrows, but they rarely fight, any disputes being settled by ritualistic displays. Their burrows can be over 330 ft (100 m) from the sea and over 3 ft (1 m) deep.
RITUAL DISPLAY Each species of fiddler crab waves its claw in a slightly different way. If this ritual movement does not deter a rival male, then two crabs may “arm-wrestle” each other to resolve their dispute. The weaker individual usually retreats before any serious damage is done. LEFT- OR RIGHT-HANDED?
Both crabs in this picture are righthanded, but in some males it is the left claw that is enlarged (see below).
OCEAN LIFE
RESTORING THE BALANCE
On arriving at the shore, the crabs head to the ocean, where they replace water and body salts lost during the arduous journey down from the forest plateau.
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Red Crab Migration TRAVELING ACROSS LAND
STAGES OF MIGRATION
RELEASING EGGS
The remarkable annual migration of the red crab (Gecarcoidea natalis) from the forests of Christmas Island (southwest of Indonesia) down to the sea to spawn is one of the wonders of the natural world. Until recently, about 120 million of the crabs made the journey each year, spending the rest of the time on a forested plateau about 1,200 ft (360 m) above sea level. Red crabs are mainly herbivorous, feeding on fallen leaves, fruit, and flowers gathered from the forest floor, but they also eat other dead crabs and birds when the opportunity arises. They conserve water by restricting their activity to times of high humidity (over 70 percent), retreating to their burrows during drier periods. Like their marine counterparts, red crabs have gills for respiration, but the gill chamber of this species is lined with tissue that acts as a lung and maximizes gaseous exchange. Red crabs on Christmas Island have undergone a noticeable decline in numbers, largely due to the accidental introduction of the yellow crazy ant in the 1930s. Since the mid-1990s, about 20 million red crabs are thought to have been killed by the ants, which squirt formic acid on the crabs as a defense mechanism when they are disturbed. There is also pressure to increase the number of phosphate mines on the island, which would involve deforestation, depriving the crab of its habitat in the affected areas.
MAN-MADE OBSTACLES Although the distance to the shore is only about 1,600 ft (500 m), red crabs have to negotiate a number of obstacles, including roads and hot train tracks. In the past, it was not unusual for one million crabs to perish each year, yet this had little impact on the population. Today, various measures such as road closures and concrete underpasses offer some security to the crabs, but they still run the risk of dying from dehydration during their journey. EGG RELEASE After incubating up to 100,000 eggs for 12–13 days, the females leave their burrows and gather on the shore to disperse them directly into the sea. They do so at night, as the high tide turns, by raising their claws and shaking their bodies vigorously to free the eggs from their pouches. Crabs on the cliffs may be 25 ft (8 m) above the water.
MEGALOPAE LARVAE Red crab eggs hatch as soon as they hit the water. The young remain in the ocean for up to 30 days, and pass through several larval stages before returning to the shallows as shrimplike megalop larvae. They metamorphose into tiny crabs after 3–5 days and leave the water to start life on land.
The onset of the wet season on Christmas Island, usually in early November, signals the start of the red crabs’ migration, which takes place over three lunar cycles. The males set off first, followed by females. It takes about a week for the crabs to reach the shore. After dipping in the ocean, the males compete fiercely for space to dig their burrows, where it is thought mating takes place. The males then return inland, leaving the females to brood their eggs in the burrows. They emerge after about two weeks to release their eggs into the sea at high tide at night during the last quarter of the moon. Phases of the moon over a 3-month period from November to January
full moon
FROM SEA TO LAND
The Sequence of Migration
new moon females and males males females
in early November, the crabs start to move down from forest plateau to the shore to breed
females return to the forest
in the second and largest migration, the crabs dip in the sea to replenish body salts, then the males fight to establish burrows
after mating, males dip a second time, then either return inland immediately or remain and feed
females move to the shoreline, releasing eggs into the sea at the turn of the high tide during the last quarter of the third lunar cycle
OCEAN LIFE
young crabs emerge after about a month, having matured in the water
first wave of crabs reaches the shoreline
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ANIMAL LIFE SUBPHYLUM HEXAPODA
SUBPHYLUM HEXAPODA
Shore Bristletail
Rock Springtail
Petrobius maritimus
Anurida maritima
DISTRIBUTION
LENGTH
LENGTH
2/ 5
Up to 1/8 in (3 mm)
in (1 cm)
HABITAT
HABITAT
Rocky shores in the splash zone
Upper intertidal zone of rocky shores
British Isles excluding Ireland
The Shore Bristletail, also known as the Sea Bristletail, derives its common name from the three long filaments extending from the tip of its abdomen. Its long body is wellcamouflaged by drab-colored scales. It has long antennae and compound eyes that meet at the top of its head. The shore bristletail lives in rock crevices and feeds on detritus. It can move swiftly around the rocks using small spikes on its underside, called styles, to help it grip. When disturbed, it can leap small distances through the air by using its abdomen to catapult it away from the rock.
DISTRIBUTION
Coasts of the British Isles
At low tide hundreds of rock springtails wander down the beach searching for food, returning to the shelter of their rock crevices an hour before the tide turns.Vast numbers
in (1 cm)
HABITAT
Sand dune systems
Marine Skater
Kelp Fly
Halobates sericeus
Coelopa frigida
LENGTH
LENGTH
Females: 1/5 in (5 mm)
1/ –2/ 8 5
HABITAT
HABITAT
Ocean surface
Temperate shores with rotting seaweed
OCEAN LIFE
DISTRIBUTION Pacific Ocean between 40º and 5º north and south of the equator
This is a member of the only truly marine genus of insects. The marine skater spends its entire life on the surface of tropical and subtropical oceans where winter temperatures rarely fall below 68ºF (20ºC). Little is known about these insects due to the difficulty in studying them. Females are larger than males and after mating they lay 10–20 cream-colored, oval eggs on a piece of flotsam, such as a piece of floating wood. The eggs hatch into nymphs that molt through five stages before becoming adults. Because this insect never dives below the surface, its diet is restricted to small organisms such as floating fish eggs, zooplankton, and dead jellyfish. It feeds by releasing enzymes onto the surface of its food and then drawing in the predigested material through its modified mouthparts.
DISTRIBUTION
shorelines
in (3–10 mm)
North Atlantic and north Pacific
LENGTH
Up to 3/4 in (2 cm) HABITAT
Intertidal sandy and muddy shores Coasts of the British Isles and northern Europe
DISTRIBUTION
in seawater they simply float up to the surface and fly off. Their larvae are equally waterproof. Strongly attracted to rotting seaweed by its smell, the female kelp flies seek out warm spots in which to lay their eggs. The larvae hatch and feed on the seaweed around them. After three molts they pupate; the adults emerge and complete the life cycle about 11 days after the eggs were laid. Kelp flies are an important food source for several coastal birds, including kelp gulls and sandpipers.
LENGTH
SUBPHYLUM HEXAPODA
Bledius spectabilis
The most widely distributed of the seaweed flies, the kelp fly is found almost everywhere there is rotting seaweed along a strand line. They have flattened, lustrous black bodies, tinged with gray, and bristly, brownish yellow legs. Of the two pairs of wings, only the front pair is functional, the hind pair being modified to small clubshaped halteres that act as stabilizers when in flight. Kelp flies can crawl through vast layers of slimy seaweed without getting stuck, and if immersed
Osmia aurulenta
Important in the pollination of sand-dune plants, the dune snail bee has a compact, brownish black body with a dense covering of golden red hairs that later fade to gray. Unlike the honey bee, which carries any pollen it collects in pouches on its legs, the dune snail bee carries its pollen in a brush of hairs under its abdomen.
Intertidal Rove Beetle
Male bees of this species emerge between April and July, a little earlier in the year than the females, and seek out territories that contain a snail shell. They then leave scent marks (pheromones) on the stems of plants to attract passing females. Once a female has mated with her chosen partner, she will adjust the position of the shell so that the entrance is oriented in the most sheltered direction and lays her eggs inside it.
Dune Snail Bee 2/ 5
SUBPHYLUM HEXAPODA
Unusual in that it lives in the intertidal zone after which it is named, this small arthropod has an elongated, smooth black body. Short reddish brown wing cases, or elytra, protect the wings but leave most of the flexible abdomen exposed. A mobile abdomen allows the intertidal rove beetle to squeeze into narrow crevices and also to push its wings up under the elytra. Most rove beetles are active either by day (diurnal) or by night (nocturnal), but the life of the intertidal rove beetle is dictated by the tides. It builds a vertical, wine-bottle shaped burrow in the sand with a living chamber about 1/5 in (5 mm) diameter and retreats into it whenever the tide comes in. The burrow entrance is so narrow—about 1/10 in (2 mm) in diameter—that the air pressure within prevents any water from entering. The female lays her eggs in side chambers within the burrow and remains on guard, until her offspring have hatched and are mature enough to leave and construct their own burrows.
SUBPHYLUM HEXAPODA
DISTRIBUTION Coasts of northeastern Atlantic, North Sea, Baltic, and Mediterranean
SUBPHYLUM HEXAPODA
of them squeeze together in the fissures to avoid being immersed at high tide. It is here that they molt and lay their eggs safe from submersion and many of their predators. Rock springtails are blue-gray in color with segmented bodies that are wider at the posterior end. They have three pairs of appendages used for locomotion, which also allow them to swarm over the surface of calm rock pools without sinking—they cannot swim. Springtails are so named for their jumping organ, called the furcula, which acts like a spring, propelling the animal upward if threatened. Unlike other springtails, however, the rock springtail does not have this feature.
BRYOZOANS ATTACHED TO SEAWEED
Bryozoans
Bryozoans, such as this hard species encrusting a seaweed, often live in areas with strong currents.
THESE COLONIAL ANIMALS live
DOMAIN Eucarya KINGDOM Animalia
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attached to the sea bed and although SPECIES 6,085 numerous, they are often overlooked. The individuals making up the colony are usually less than 1/32 in (1 mm) long, but the colonies may span over 3 ft (1 m). Bryozoans are also called ectoprocts or sea mats, the latter name referring to their tendency to encrust the surfaces of stones and seaweeds. Other colonial forms of bryozoan include corallike growths, branched plantlike tufts, and fleshy lobes. Most species are marine, but a few live in fresh water. PHYLUM Bryozoa
Anatomy
Habitats
A bryozoan colony is made up of individuals called zooids, and may contain several or up to many millions. Each zooid is encased in a box-shaped body wall of calcium carbonate or a gelatinous or hornlike material, and a small hole links it to other zooids. To feed, the animal pushes a circular or horseshoe-shaped structure (a lophophore) out of an opening. This is crowned by tentacles covered in tiny, beating hairs that draw in planktonic food. In most species, fertilized eggs are stored in specialized zooids that form a brood chamber for developing larvae.
With their great variety of body form, bryozoans can be found in almost any habitat from the seashore to the deep ocean, and from Arctic waters to tropical coral reefs. Colonies are most often found firmly attached to BRYOZOANS UNDER ATTACK submerged rocks, seagrasses, seaweeds, Sea slugs often make a meal mangrove roots, and dead shells, but some of encrusting bryozoans, encrusting species even hitch a ride on the breaking into each zooid shells of living crustaceans and mollusks. A few and eating the insides. unusual species do not need a surface for support and can live in the sand; these bryozoan colonies can move slowly over or through the sand by coordinated rowing movements of a long projection found on specialized zooids. Bryozoan colonies originate from a single larva that settles on the seabed and becomes a zooid. More zooids are added to the colony by budding, a process in which a new zooid grows out from the side of the body wall. Most bryozoan larvae are short-lived and settle near the parent.
MAT OF ZOOIDS
This encrusting species of bryozoan has rectangular zooids joined in a single layer. The resulting mat spreads over seaweeds.
ORDER CTENOSTOMATIDA
ORDER CHEILOSTOMATIDA
Gelatinous Bryozoan
Hornwrack
Alcyonidium diaphanum
Flustra foliacea
DISTRIBUTION
SIZE (HEIGHT)
SIZE (HEIGHT)
Up to 12 in (30 cm)
Up to 8 in (20 cm)
DEPTH
DEPTH
0–656 ft (0–200 m)
0–330 ft (0–100 m)
HABITAT
HABITAT
Rocks and shelly sand
Stones, shells, rock
Temperate waters of northeastern
Atlantic
Temperate and Arctic waters of northeastern Atlantic
DISTRIBUTION
This species is often mistaken for a brown seaweed. The colony grows up from a narrow base as thin, flat, fan-like lobes. These usually form dense clumps and cover the sea bed like a crop of tiny brown lettuces. They litter the strandline on many shores in dried clumps and, by using a magnifying glass, an observer can easily see the individual, oblong colony members.
Pink Lace Bryozoan Iodictyum phoeniceum SIZE (WIDTH)
Up to 8 in (20 cm) DEPTH
50–130 ft (15–40 m) HABITAT
Rocky reefs DISTRIBUTION
Australia
Temperate and tropical waters around
Pink lace bryozoan colonies feel hard and brittle to the touch because the walls of the individual zooids are reinforced with calcareous material. The colony is shaped like curly-edged potato chips with a lacework of small holes.The beautiful dark pink to purple color remains even after the colony is dead and dried. This species prefers to live in areas with some current, and its holes may help reduce the force of the water against it. Similar species are found on coral reefs throughout the Indo-Pacific region.
OCEAN LIFE
Colonies of this bryozoan have a firm, rubbery consistency and grow as irregular, lobed, or fingerlike growths that attach to their substrate with a small, encrusting base. This species may cause an allergic dermatitis when handled, and North Sea fishermen are often affected when their trawl nets have gone through areas of dense bryozoan undergrowth.
ORDER CHEILOSTOMATIDA
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ANIMAL LIFE
Echinoderms THE NAME OF THIS PURELY MARINE
group of invertebrates is derived from the Greek for “hedgehog skin.” The group KINGDOM Animalia includes starfish, sea urchins, brittlestars, feather stars, and sea PHYLUM Echinodermata cucumbers. Echinoderms have radiating body parts, so most CLASSES 5 appear star-shaped, disc-shaped or spherical, and all have a SPECIES About 7,278 skeleton of calcium-carbonate plates under the skin. Inside is a unique system of water-filled canals, called the water-vascular system, that enables them to move, as well as to feed and breathe. Typically bottomdwellers, they live on reefs, shores, and the sea bed. DOMAIN Eucarya
RADIAL SYMMETRY
This tropical starfish has the typical fiverayed structure of echinoderms. Its arms are protected by hard plates, and its bright colors warn predators of its toxins.
Anatomy The echinoderm body is based on a five-rayed symmetry similar to the petals of a flower. This is apparent in starfish, brittlestars, and urchin shells (tests). Sea urchins are like starfish, with their arms joined to form a ball. Sea cucumbers resemble elongated urchins—their five-rayed symmetry can be seen end-on. The echinoderm skeleton is made of hard calcium-carbonate plates, which are fused to form a rigid shell (as in urchins) or remain separate (as in starfish). Usually, it also features spiny or knoblike extensions that project from the body. Sea cucumbers have minimal skeletons reduced to a series of small, isolated plates. The water-vascular system consists of a network of canals and reservoirs, as well as tentacles that extend through pores in the skin to form hundreds of tiny tube feet. outlet of water-vascular system (madreporite)
anus
tube foot spine
gonad
calciumcarbonate plate
watervascular canal
intestine
mouthparts
MINI SUCKERS
Tube feet act like hydraulic suckers. They are operated by water squeezed in and out from a small reservoir similar to the bulb on the end of an eye dropper.
SEA URCHIN BODY PLAN
The body consists of a fluid-filled cavity inside the shell (test), which houses the organs. The mouth is in the centre of the underside, and the anus is on top of the upper side.
OCEAN LIFE
ECHINODERM CLASSIFICATION The echinoderms are split into five classes based on their shape, skeleton, and the position of their mouth, anus, and madreporite. A sixth class, “the Concentricycloidea,” was at one time set up for the newly discovered sea daisies but these two species of disc-shaped animals are in fact a strange type of starfish and are now regarded as members of the class Asteroidea.
FEATHER STARS, SEA LILIES Class Crinoidea About 638 species
Also known as crinoids, these animals have a saucer-shaped body extending into five repeatedly branching, feathery arms used as filter-feeding appendages. Mouth and anus face upwards. Sea lilies attach to the sea bed by a jointed stalk, but feather stars break free when young to become swimming adults. STARFISH OR SEA STARS Class Asteroidea About 1,851 species
The body of these mostly seabed scavengers is star-shaped, with five or more stout arms
merging into a central body disc. On the underside of the arms are rows of numerous tube feet and a groove, along which they pass food to the central mouth. The mouth is on the underside, and the anus and madreporite are on the upper surface. The skeleton is a layer of plates (ossicles) embedded in the body wall.
SEA URCHINS, SAND DOLLARS Class Echinoidea
BRITTLESTARS, BASKET STARS Class Ophiuroidea
SEA CUCUMBERS Class Holothuroidea
About 2,074 species
About 1,716 species
These echinoderms have a disc-shaped body with five narrow, flexible arms. Basket stars’ arms are branched and finely divided. The skeleton is a series of overlapping plates. The mouth, on the underside, doubles as an anus.
These echinoderms have a sausage-shaped body with five double rows of tube feet, with those encircling the mouth modified into feeding tentacles. The skeleton comprises small, multi-shaped plates.
About 999 species
Body shape ranges from a disc (sand dollars) to a sphere (urchins) with five double rows of tube feet. The skeletal plates join to form a rigid shell (test) with movable spines.
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Reproduction Most echinoderms have separate males and females, which reproduce by releasing sperm and eggs, respectively, into the water. Individuals often gather to spawn at the same time, thereby increasing their chance of success. This synchronized spawning is initiated by factors such as daylight length and water temperature. Each echinoderm group has its own type of larva with its own way of swimming, floating, and feeding. Some starfish, for example, keep their fertilized eggs and developing larvae in a pouch under their mouth, and nourishment comes in the form of yolk. In some brittlestars, the larvae are brooded in sacs inside the body, and the young are released after metamorphosis. In most species, however, the fertilized eggs drift in the plankton and develop into free-floating larvae. The larvae eventually transform into their adult form and settle on the sea bed.
FLOATING AIDS
Long, paired arms help sea-urchin larvae, such as this one from a sea potato or heart urchin, to float in the plankton. Brittlestars have similar larvae.
RELEASING SPERM AND EGGS
By rearing up to spawn, sea cucumbers ensure that their eggs drift away to mix with sperm released by another individual.
Feeding Echinoderms range from peaceful grazers and filter feeders to voracious predators. Carnivorous species of starfish extend their stomach over their prey and digest it externally. In contrast, most sea urchins are grazers, scraping rock surfaces using teeth that resemble the chuck of an electric drill. Combined with muscles and skeletal plates, they form a complex, powerful feeding apparatus called Aristotle’s lantern. Sea cucumbers have an important role as sea-bed cleaners, vacuuming up organic debris and mud.
FILTER FEEDING BY TUBE FEET
Feather stars raise their arms to trap plankton using fingerlike tube feet. The food is trapped in mucus and passed down the arms into the mouth.
Defense
HUMAN IMPACT
PEDICELLARIAE
Slow-moving urchins and starfish can become overgrown by planktonic larvae looking for a place to settle. They defend themselves by using spines modified into tiny pincers, called pedicellariae, to catch and crush the larvae.
FIGHTING OVER OYSTERS In European waters, the common starfish has a voracious